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 keep smiling,