Epoxy School Blog

Epoxy troubleshooting - soft patches or "hot spots"

Have you ever seen a solventless, two-pack epoxy floor coating cure hard except for a couple of soft patches, or “hot spots” as they’re sometimes called?

Do you know why something like this might happen? There are a few reasons, however one of the most common causes for mistakes like this is simply a failure to mix the coating correctly.

Soft spots, or soft patches, create a dirty looking floor.

Mixing well isn't enough

OK, you may have beaten the product with a drill mixer for five minutes, but unfortunately that doesn’t mean you mixed it well. The fact is you will always have areas within your bucket that do not mix well naturally – namely, the sides – and if you don’t pay attention to these then hot spots can occur.

I see and hear about it all the time; a contractor has mixed the product in a bucket and scraped the sides or left the bucket sitting upside down to get as much out as possible. There’s nothing wrong with trying to minimise losses this way, however to avoid soft epoxy patches you must be sure these regions are mixed properly as well. The only way to do this is to scrape the sides of the bucket during the mixing process to lift the stagnant product and draw it into the middle. It becomes even more important during cooler temperatures or if using a thicker product that doesn’t flow as easily.   

What about larger soft spots?

The hot spots created by poor mixing practices with solventless epoxies are usually small in size, so what might be the cause of larger soft spots?  If you have large soft spots, i.e. covering a full kit or more, then I would be asking questions about the mix ratio or, heaven forbid, forgetting to add the curing agent at all. Don't laugh, it does happen! If the coating was a solvent-borne or water-borne product (or even if solvent was added to a solventless product) then something called solvent entrapment might be in play and I’ll get around to that in another post.

How can I fix soft spots?

Unfortunately there are no quick fixes for soft spots. You can’t simply roll some curing agent into the mix and hope it magically hardens into the perfect film you originally intended. Instead, you have to take your medicine, so to speak, look to remove the defective film and go again. On most occasions this will involve scraping up the soft epoxy and solvent wiping away any residue. Once the solvent has evaporated you can patch the area or overcoat it. Be aware though, if you go back to bare concrete with your solvent wipe then you should patch that immediate area first before over-coating otherwise you could end up seeing it through the topcoat.

Take care and talk to you later,

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Surface preparation - should I use a primer or sealer?

Some call them primers and some call them sealers, but are these terms interchangeable?

For me, the answer is no. An epoxy primer serves a different purpose to an epoxy sealer and if you’re not careful making that distinction it could come back to bite you. The fact is, primers won't always be suitable for sealing and sealers won't always be suitable for priming. The key to avoiding problems is simply asking why you are priming or sealing in the first place.

Primers increase adhesion

In my opinion, a primer is used for its adhesive qualities, meaning the next coat doesn’t have sufficient adhesion or is not compatible with the surface it’s bonding to. An everyday example would be using a primer before trowelling a 6:1 mix of resin and sand. At 6 parts sand to 1 part resin, there’s barely enough resin to hold the sand together let alone provide a good adhesive layer and priming would therefore be beneficial.

Sealers are for sealing

A sealer, on the other hand, serves the purpose of closing off the substrate before you apply the first coat. You can apply a sealer for a number of reasons, including the prevention of defects like pinholes or to improve the finish of subsequent coats.

Another difference between the two is that you tend to only require a single primer coat whereas you can require multiple sealer coats. For example, you apply the first sealer coat and notice it looks sealed/glossy in some areas, but bone dry in others. If I was concerned about defects, I’d want to see a consistently sealed surface before the first coat went down.

Sealing a concrete slab with a clear epoxy sealer.

Do I have to prime or seal?

A big question that comes into play on this topic is whether you always need to prime or seal. Once again, the answer is
no. You can buy primerless products that are resin rich enough and surface tolerant enough to be applied directly onto the substrate, eliminating the extra labour and product required to apply a primer. You can also get surfaces that aren’t weak or porous and won’t need a sealer to bring them up to scratch.

Finally, is it worth priming and/or sealing “just in case” to guarantee better results? You guessed it – no, not always. I can give you an example where sealing actually caused the trouble. In this case the contractor wanted a sealer to strengthen porous concrete, however in doing so they compromised the adhesion and caused some big headaches for themselves. It’s a very interesting story that I’ll have to cover in more detail at another time.

Take care and talk to you later,


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Surface preparation - to over-coat or not to over-coat

There’s a job coming up with an existing floor coating and the client would prefer to over-coat rather than remove to save money. What do you do?

There’s no right or wrong answer to the question, just risks you need to weigh up in deciding to over-coat or not.

Picture of an existing coating you wouldn't want to over-coat.

The over-coat risk checklist

Here the checklist I use when deciding to over-coat

  1. Do you know what the existing coating is? Is it a two-pack or single-pack coating? If you put a little bit of solvent on it does it bubble or wrinkle? If it doesn’t handle solvent well then it’s probably a single-pack coating, which means you shouldn’t waste your time over-coating with the much stronger two-pack.
  2. Is the existing floor coating stuck down well or flaky? If it’s flaky then remove it. Over-coating a flaky coating could end up with both peeling off.
  3. Is the existing coating stained, meaning that contamination has absorbed into the coating? This is not a good sign and if the stain is oil based then I’d remove the coating.
  4. Is the existing coating bonded well, but badly worn? This is one of those 50/50 scenarios where some will err on the side of caution and just remove it while others will over-coat.
  5. Is the new floor coating subject to harsh conditions, either through chemical exposure or temperature fluctuations? If so then I’d want to ensure it had the best possible chance of withstanding the conditions by bonding it directly to the substrate.
  6. Does the new coating wet out the existing coating? For example, if you do a small test patch does the coating flow nicely, or does it crawl and cause surface defects? Obviously this sort of incompatibility means the existing coating would need to be removed.
  7. If it’s a large project and you’re completely unsure on over-coating, can you do a small trial? That is, apply the coating as you intend to on a larger scale to see if it all works.
  8. A practical consideration is: can you even remove the existing coating within the shutdown period? Not every coating grinds off easily and this may override all other considerations.

Involve the client

If you choose to
over-coat at the end of the day, be sure to educate the client on what you are doing and any risks that might be involved. Be open and honest so they can participate in the decision-making process. I’d much prefer the client tell me they’re prepared to take the risk than wear all the responsibility myself. Overall, if there’s too much doubt around over-coating then you’re much better off to remove the existing floor coating regardless of what the client says. If you lose the job because you’re now too expensive then so be it. You never make money when you have to re-do a job anyway!

Take care and talk to you later,


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Manufacturer's secrets revealed - re-worked product

What is re-worked product and why do some epoxy manufacturers cringe when people talk about it?

Re-working product is a practice some epoxy manufacturers use in the coating industry to dispose of a faulty batch. The idea is if a batch doesn’t fall within specifications, you can gradually get rid of it by adding as much as possible into the next batch.

How tight is tight enough?

Sounds logical enough I suppose, but in my opinion the practice isn’t a good one. As an epoxy manufacturer you should have tight specifications to ensure product consistency and if it allows for re-worked product then I would argue that specification is too loose (which, by extension, makes a faulty batch look even worse).

In my humble opinion, a product should be made the same way every time. If it’s allowed to vary too much within the manufacturing specifications then differences could start to show up in the application, curing or performance from one batch to the next. That spells one thing – trouble!

A Brookfield Viscometer is used in quality assurance for coatings.

Quality assurance not always a guarantee

So, is the practice of re-working product limited to small manufacturers? Surely a larger, quality-assured epoxy manufacturer wouldn’t re-work product? Right? Wrong!

Quality assurance systems are only there to ensure a product is delivered within a listed set of parameters every time. The manufacturers set how tight the specification parameters are to begin with and the procedures that follow a failed batch, so as long as they stick to their own rules they’re ok.

How to handle out-of-spec product

If re-working product isn’t the answer then what is? What should be done with a failed batch?

Instead of looking for a band-aid solution like re-working, you’ve got to firstly ensure the manufacturing process is tightly controlled and that preventable errors are not appearing in the first place. If a batch still fails for some reason, label it as such and find a project that can use the out-of-spec product as it is. This obviously means more work for the manufacturer, but if you know the project and the contractor is also onboard then you can still have a successful outcome.

Re-worked product is one secret that will make some epoxy manufacturers cringe...and I dare say some epoxy users too. Product substitution is another one that fits into the same category, but more on that some other time perhaps. Finally, if re-worked product doesn’t agree with you either, how can you find out if a manufacturer does it? Simply ask them and seek an answer in writing.

Take care and talk to you later,


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Selling epoxies - why should we talk volume and not weight?

Why do I prefer talking about coverages in volume and not weight as many others do?

This is only a short post, but I feel it helps clear up and very common point of confusion in the industry.

The Golden Rule of coverages

The basic laws of physics give us a very simple rule that we can use to derive coverages. I call it the Golden Rule and it goes like this: 1 litre will cover 1 square metre at 1mm thick.

Using this rule, you can derive the coverage of a product based on a film thickness. For instance, you can work out that 1 litre applied 500 microns will cover 2 square metres, or, with a little more effort, you can turn the equation around to work out a 300-micron film applied over 45 square metres will require 13.5 litres.

Is it really cheaper?

With the simplicity and power of this rule, why do some start talking about weight when it comes to product pricing?

There might be a few answers to that, but the most obvious reason I can see is marketing. As most products have a density higher than 1, it sounds cheaper to sell based on product weight. For example, let’s say a product costs $30/litre and has a density of 1.5kg/L – if sold per kilogram it would “only” cost you $20/kg. It sounds like a bargain on the surface, but when working with actual coverages then it’s all the same at the end of the day.

There is always a “but”

A couple of points I should add on this –

  1. The coverage rule only applies in its simplest form if there’s no evaporative solvent in the resin (see previous post on solventless resins). You can still use it, but things become a bit more complicated when solvents are involved because the dry film thickness isn’t the same as the wet film thickness.
  2. The coverage rule relates to theoretical coverage only. During application you’ll experience losses from a number of factors such as wastage in the roller/brushes and residual product left in mix containers.

For the contractors working with imperial measurements – square feet, gallons, mils etc. – I’m afraid there’s no simple coverage rule equivalent to the metric one. You’ll have to stick with the coverages that your manufacturer gives you and just keep in mind whether they are theoretical or practical.

Take care and talk to you later,


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Epoxy review - water-borne epoxies v solvent-borne epoxies v solventless epoxies

I’ve been asked countless times to explain the differences among water-borne, solvent-borne and solventless epoxies and what it all means as far as their suitability for certain flooring applications.

They’re all epoxies of course, what does change is the “delivery” of the resin. For some quick definitions –

  • Water-borne epoxies are epoxies dispersed in water, which evaporates from the film after application.
  • Solvent-borne epoxies are epoxies dispersed in organic solvent, which evaporates from the film after application.
  • Solventless epoxies are epoxies that have nothing evaporating from the film after application.

Water-borne epoxy being cleaned up with water.

The key question is how do those basic differences impact on which one I choose?

There are all sorts of reasons why an epoxy user might choose one over the other; each type has advantages and there’s not one product for all applications. However, for concrete flooring applications this is the way I see it.

Solvent-borne epoxies

Solvent-borne epoxies are the traditional epoxy. The solvents allow the coating to apply easily in thin films and deal with any formulating problems like surface tension. Originally designed to protect steel, it was later used for concrete structures and has been adapted for flooring.

Its viscosity and relative ease of use (including long pot life) make it an adaptable type of coating where thin films are required, however the product can have some nasty aspects, e.g. solvents and solvent entrapment. Also, I don't believe 150 microns/6 mils total dry film thickness provides a long term flooring solution when the profile of concrete itself can vary just as much.

Water-borne epoxies

Water-borne epoxies were first used as moisture barriers to deal with hydrostatic pressure in concrete slabs. They’re characterised by a smooth, creamy texture and are effortless to roll out. Being water-soluble, they can easily penetrate into the slab pores and harden, blocking off the pores moisture travels through.

As water-borne epoxies are so nice to apply there’s a natural temptation to use them as floor coatings, but there are some inherent problems when doing so. The most pressing is the inferior film strength when compared to the other two epoxy types. Also, because water is slower to evaporate than organic solvents there’s a greater risk of solvent entrapment issues. A classic example being the thicker regions of a film created when over-coating divots; the epoxy starts to crosslink before the water can escape and leads to softer, weaker spots.

Water-borne epoxies are also considered by many as a better environmental alternative, but being water based does not necessarily mean it has less impact on the environment. That is a whole new topic that I might need to cover later!

Solventless epoxies

Solventless epoxies
– also called solvent-free epoxies or 100% solids epoxies – have the performance of solvent-borne epoxies in many ways, but do not contain evaporative solvent. Without the solvent, the viscosity is quite different and it applies quite differently. If you are used to either solvent-borne epoxies or water-borne epoxies and you wanted to change to solventless, I would spend some time talking to the manufacturer and playing with some samples. The capabilities of solventless epoxies are tremendous and they’re very versatile, but don’t think they can do the job of a water-borne epoxy for hydrostatic pressure or go down at less than 100 microns like a solvent-borne epoxy.

As I said before, each technology has its own advantages. For concrete flooring, I will generally only talk about two-pack solventless epoxy technology as it is the most versatile across a range of epoxy applications such as decorative, commercial, non-slip, industrial and heavy industrial.

Take care and talk to you later,


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Epoxy troubleshooting - bubbles, pinholes and craters

Why do some epoxy users get bubbles, pinholes and craters in concrete coatings while others don't?

Pinholes in a concrete coating film.

I guess the answer to that is part education, part habit and perhaps even part luck! In other words, those that don’t suffer know their causes, practice the right habits to prevent them and, in some cases, just have a bit of good old-fashioned luck.

Bubbles, pinholes, craters - education

The education part comes from understanding what causes bubbles, pinholes and craters in concrete coatings.

It all has something to do with the fact concrete is porous. Granted there are different degrees of porosity, but the reality is they are all porous on the scale we’re considering here. With a porous substrate, the voids are filled with air that expands when heated and contracts when cooled. When the air expands it pushes out of the slab through the path of least resistance, which in this case is the top, especially if we’ve “opened up” the slab through grinding or other mechanical means.

To get to the bottom of why these defects occur in a coating film, we need to consider what happens when a concrete coating is applied and air is being expelled in this manner. Whilst the coating is wet, the air being pushed through will blow a bubble – what happens after that point depends on how much air is coming through and whether the bubble pops or not. If the concrete coating has hardened sufficiently and the bubble doesn’t pop, you are often left with a nice complete bubble. If the bubble pops after the concrete coating has gelled and can’t flow anymore, it will leave a pinhole. Finally, if a large bubble pops after the concrete coating has gelled, it will leave a crater (sometimes with a loose “skin” attached).

Bubbles, pinholes, craters - habits

The habit part relates to routinely doing whatever you can to prevent these defects from happening. Two very effective measures are: applying only on the cooling cycle of the slab, i.e. when the air is cooling and drawing in rather than expanding out, and, using a sealer coat to block the voids before applying the concrete coating. The air will still be expanding and contracting, but with a sealed top the path of least resistance is no longer through the surface.

Just as a side note on sealing, be aware applying one thin film across the whole floor doesn’t guarantee all the voids are closed off. If some areas still appear dry then there could still be the potential for air to escape, so you might look to seal again or until the slab at least looks more even.

Bubbles, pinholes, craters - luck

Finally, the naive luck side of things refers to the small percentage of users that have no idea why bubbles, pinholes and craters happen, yet they seem to avoid them most of the time. They apply concrete coatings at all times of the day, onto slabs with all sorts of porosity, however they somehow get through largely unscathed. Some days they might get pinholes and other days they might not. Who knows? While this might be frustrating for others that aren’t so lucky, rest assured it will catch up with them sooner or later! If you’ve only relied on luck in the past, I hope you now have the education and the habits to help you control your bubble, pinhole and crater destiny.

Take care and talk to you later,


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Epoxy troubleshooting - hot tyre pick-up

Have you ever seen a garage floor coating lift right where a car was parked?

You might not see it on all tyres; you might not even see it the first few times the car parks on the garage floor. If it does happen though, it could be a case of the dreaded hot tyre pick-up and it more than likely had something to do with a few common causes.

Car parked on a garage floor.

Hot tyres and garage floor problems

The reasons why garage floors can lift under a tyre are –

  • The preparation of the concrete was poor and laitance was not removed properly before coating. It takes very little effort to fail a coating on a poorly prepared slab.
  • The garage floor coating was applied onto a previously uncovered slab with oil or silicone contamination (over-spray from silicone “tyre shine” products is a common source of silicone contamination). If there was such contamination on the concrete then typically crawling would’ve been seen after basecoat application.
  • The basecoat was a single-pack product with low crosslink density and poor heat resistance. When the hot tyre parked on top, the heat transfer softened the film and made it easier to peel off. 
  • The basecoat may have been a two- pack water-based product, but it didn’t crosslink fully and resulted in a porous, relatively weak film. This can happen if the product is applied too thick and starts to crosslink before the water has evaporated out of the system.

The typical scenario

With these common reasons explained, you can probably now piece together what usually takes place in hot tyre pick-up situations.  Firstly, the garage floor coating was probably applied without properly cleaning the slab or thoroughly preparing the surface beforehand. The product used was also pretty cheap and put down by an inexperienced set of hands without closely observing all the instructions. The finish looked ok, however when it came in contact with hot tyres the heat softened the film and created a peeling stress that pulled it off once the tyres had cooled.

Avoid the peel

So there’s the typical problem in a nutshell, but how do you overcome it? In light of what I’ve said about the causes, doing the following three things would be a good start –

  1. Do water bead tests to check if there’s any contamination before commencing the coating process. Remove the contamination first and then thoroughly prepare the surface to establish a profile for the garage floor coating to grab onto. 
  2. I favour two-pack epoxy products as the basecoat because they tend to have greater overall durability. In addition, I favour two-pack solventless epoxy products because I don’t like solvents inside and if it’s moisture tolerant as well then I minimise my risk of other curing problems.
  3. Finally, use the products as recommended by the manufacturer, paying particular attention to the curing conditions, film thicknesses etc.
  4. To finish, the best example I ever heard of hot tyre pick-up was a water-based epoxy product used in a garage that didn’t pull off in a large chunk, but rather a very neat-looking tread pattern! It was an amazing thing to see, although I doubt the owner would’ve been so thrilled!

What other classic garage floor problems have you come across?

Take care and talk to you later,


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Patching concrete floors - is a "bog" good enough?

You strive for the best flooring system possible, so why perform concrete repair with any old “bog”?

Perhaps “bog” is a slang term from my part of the world, but generally speaking it’s a cheap patching compound used to fill cracks and voids before over-coating. The timber industry has a bog, the auto industry has a bog, the building industry has a bog – but, in my opinion, the concrete flooring industry definitely should not have a bog! Let me explain why.

What is a bog?

A bog is typically a highly filled, fast-cure product with very little resin in the mix. This makes it cheap, but prone to inconsistent adhesion and that’s exactly what you don’t want in a flooring environment. So what type of patching compound should you use?

A resin-rich epoxy patching compound being applied into a divot.

Patching compound wish list

In my opinion, I want to do
concrete repair with a product that has the following properties –

  • Resin rich – a two-pack epoxy system that is rich in resin so I don’t have to worry about adhesion. When applying patching compounds, you find they tend to dry out as you scratch and scrape it across the concrete. If you start with a compound that’s dry to begin with, it will quickly become unworkable and the adhesion even more of a concern. 
  • Ready to mix – a pre-formulated, ready-to-use patching compound eliminates the inconsistencies adding in bits and pieces onsite can introduce.
  • Thickness range – a patching compound that be high build or feather edge and maintain its shape regardless. Being a high-build product, I wouldn’t want any solvent or water in it as you could end up with solvent entrapment.
  • Working time – a longer standard working time is appreciated because patching can be a slow process, with the option of fast or slow cure a nice bonus.
  • Can be sanded – while sanding afterwards isn’t the aim, I’d prefer to have something that can be sanded if required.
  • Compatible – the patching compound would be totally compatible with my basecoat, e.g. didn’t cause blushing, so I could apply it wet on wet rather than having to wait for the patch to harden or dry.
  • Tintable – although not critical, I would also prefer the patching compound to be tintable in case patching was required between coats and there was a chance it could show through.

Do you have a good reliable patching compound available to you? Perhaps you have had to build your own patching compound?

Take care and talk to you soon,


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Gloss or semi-gloss floor - which one to use?

When do I recommend a full-gloss over a semi-gloss floor  or vice versa?

Sounds like a simple question and many people will just tell you, “let the client decide.” There’s nothing wrong with this approach as long as you can educate them effectively on the differences between the two and why one may be more suitable than the other. Below are a few points you’ll need them to consider.

Full-gloss and semi-gloss floor in a restaurant.

What you expect with a full-gloss floor

Looking at the fundamentals, a full-gloss floor will –

  • Tend to be resin rich so it can flow to produce the smooth, gloss floor. Being resin rich also means that it might well be more expensive per litre than a semi-gloss floor.
  • More readily show defects in the floor, so getting the substrate flat and keeping the area closed off tightly are very important.
  • More readily show porous sections in the slab where the floor coating has been absorbed.
  • May be more prone to colour separation (read post on colour separation).

What you expect with a semi-gloss floor

Conversely, a semi-gloss floor will –

  • Often be cheaper per litre as it may have more filler.
  • Not show defects as readily. You’ll be able to see divots in the slab, so you should still aim to start with a flat and even substrate, however it will be less noticeable if dust or fluff has ended up in your floor coating.
  • Easier for the client to maintain as it might not show dirt and dust as easily and will tend to conceal scuffs and scratches better.

After reading that summary you’d be forgiven for only wanting to apply the more forgiving semi-gloss floor. That might be the case, but I still think you should always come back to the original answer: let the client decide.

Costs and defects can be explained

If your client is set on a full-gloss floor, you can now at least inform them why it might cost them a little more – the product might be more expensive, the preparation needs to be more thorough and you may need to seal the slab or apply an extra coat if the slab is porous – and why it’s seen as a higher risk in terms of bugs, dust or fluff. You should also advise them that day-to-day cleaning of their full-gloss floor will take a lot more work than a semi-gloss floor.

It also goes without saying that before you offer your client a full-gloss floor you should’ve tested the product and know you can deliver the result the client is wanting. If you haven’t or you’re not sure then you’re probably better off to not take on the job.

What other reasons have you come to learn for choosing a semi-gloss floor over a full-gloss floor?

Take care and talk to you later,


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