Grain Filling Methods for Guitar Bodies

A guitar builder asks about using epoxy versus conversion varnish for filling the wood grain on his instruments. June 18, 2009

I use system three epoxy and silica to fill the grain of my work before I lay the finish over it (MLC Krystal Clear). After applying the epoxy, I let it dry for about a week then I (hand) sand it down to the level of the wood. Now, usually I sand it completely away so you don't see any epoxy left over on the surface of the wood however, when I do this, other parts of the surface go too low and the grain opens back up.

My question is, is it ok to leave little "splotches" of epoxy on the surface of the wood? It is so thin, you won't see any 'high spots' on the surface. After finishing, would I see the places where I leave the epoxy on the wood surface? Or would it disappear when I lay the finish over it?

Note that when I wet the surface with Naptha the epoxy "splotches" disappear. I am also finishing guitars and I have three of them that I'm at this stage with. It would make it much easier if I didn't have to sand it all down to the surface. One guitar is mahogany, one is black limba, and the other is Hawaiian koa.

Forum Responses
(Finishing Forum)
From contributor C:
Do you have all the materials there to try a test on scrap wood? That way you can see exactly what is happening.

From Paul Snyder, forum technical advisor:
Next time you get some Krystal, get some Level Sealer. It's made to do the job you're currently doing with the epoxy. That way you don't have to worry about witness lines or adhesion problems. Oil-base varnish works fine over epoxy, but I'm not sure about conversion varnish (Krystal). If you called the ML Campbell technical services, they'd probably recommend against it. For the three guitars you have in progress, I'd do a test as recommended. If you're buffing out your instruments, consider using a catalyzed polyurethane in place of the conversion varnish. You can get great fill with the sealer and the topcoats buff out more easily than CV.

From the original questioner:

I love how the Krystal Clear looks/sprays and its durability is amazing. I'm going to stick with it and do some test sprays. I'll also call MLC and see what they have that we can use under the Krystal Clear.

From the original questioner:
Ok, so after talking with the ML Campbell tech guy I've come up with this. For these guitars, I'm just going to machine sand (already done actually) the epoxy off. This will at least get me back to square one. To fill the grain, I'll spray the Krystal Clear every 30 minutes. Scuff sanding between each coat with 320 until the grain is completely filled. Then I'll spray a few more coats on and let it sit for three weeks. Letting it sit for three weeks will give it time to shrink back to its final state. Then we'll level sand it until it's smooth and buff it.

From Paul Snyder, forum technical advisor:
That schedule may work fine. The biggest concern I'd have is how long the cure time is going to be with that many coats applied in such a short amount of time. Three weeks may not be long enough. Also, you want to be careful not to exceed the 4-5 dry mils film thickness limit or it's likely the finish will crack. It really would make the task easier and faster to use Level Sealer. It was designed to do exactly what you want. Read the data sheet at the link below and do a good size test sample to see how you like the results.

From the original questioner:
I see what you're saying about how long it would take the Krystal to dry if it's too thick. I'm betting that that's been our biggest problem in the past (I haven't let it cure enough before we sand and buff). I'll definitely pick up a gallon of the level sealer and try it out.