Sand before priming plywood built-in

Question

I have a built-in that I made using 3/4" plywood and face-frame construction. I sanded one one section (it comes apart) and then primed it. The grain raised, as was expected. My question is, since the grain raises and I have to re-sand it anyway, do I really need to sand it before priming?

It's a sanded birch plywood, but not super smooth.

I am using Sherwin-Williams Premium Wall and Wood Primer and ProClassic Paint.

Answer 1

My question is, since the grain raises and I have to re-sand it anyway, do I really need to sand it before priming?

Birch has a tendency to be a bit stringy even after sanding, so this could be what you're experiencing.

If you're painting, you can probably get away with a less-smooth surface than if you were using a film finish like clear polyurethane. The paint will mask some of the surface irregularities a bit better. I would still do a nominal amount of sanding before painting to smooth things out a bit and give the paint a better surface with which to adhere. Then, sand between coats of primer/paint to knock back any raised grain, like you've done already.

Otherwise, one trick you can use is to "pre-wet" the surface before you sand, which will raise the grain. This way, you should be able to knock down the grain with the sanding operation and it won't re-raise when you paint.

This trick also works for other wood finishes like stain and varnish.

Answer 2

do I really need to sand... before priming?

Ideally yes. The role of sanding is not just about smoothing, it can also be about prepping a surface to properly accept a finish (or glue).

Any older wood surface should be sanded lightly prior to applying glue to ensure a good bond and the same holds for when paint or varnish is applied. Just to be clear, you can get away with not doing this in some cases but with waterbased products in particular it will give better security if you sand.

This shouldn't be any kind of heavy sanding, just a few passes with a med-fine paper is more than enough. If using a random-orbit sander finish with a pass sanding by hand (using a block) in the direction of the grain, using the same grit as fitted to the ROS.

The raised grain is a separate concern. Because the primer is raising the grain pre-wetting to raise the grain can be advisable. After this knock back the raised grain with just enough sanding as needed to remove it and no more.

So in summary here's the prep procedure you might adopt:

• sand lightly overall with 120 to 180 grit, no need to go to finer paper than this when painting;
• dampen the surface with warm water to raise the grain;
• let the wood dry thoroughly, a couple of hours if necessary;
• sand very lightly along the grain to just to remove the raised grain, using a finer paper such as 180 to 220 grit;
• prime;
• paint.