Stain Sealer Under Epoxy

I am about ready to finish the bottom of an old racing runabout. The wood is meranti and okoume plywood. I'd prefer not to stain at all, but the boat was a specific color at the point in its life that what I want to replicate. The original finish was a ureathane one step stain/varnish in a richer redder color. With just an epoxy over coat, the meranti finishes pretty gold without any stain, and the grain darkes a bit still, but not as bad as with the stain.

I found some metal complex water based dyes that are very colorfast, and won't likely fade in the very limited sunlight that the hull will see (we are talking a garage queen here, only out in the sun when it is at races). I'm well aware that an oil based stain is a bad idea.

After dyeing the wood to correct color I plan on using a light cloth that will wet out clear. I'm using Raka low viscosity resin and 6oz high clarity cloth and using the mylar technique to minimize the amount of epoxy used and the weight gain (as well as the amount of sanding and filling).

In a couple small places the ply (5mm) has some cracking damage (that was fixed by getting epoxy into the cracks) and in those spots there will be a 6oz cloth on the inside too, giving those areas a sandwich construction.

When testing the finishing, I am gettin more graining effect than I would like, the stain is darkening the the grain too much, but the overall color is good. I am doing test pieces with different stains and then overcoating with epoxy. I am thinking that very light coat of sealer would help even the stain and keep the grain from getting as black.

I realize that a stain sealer will prevent the epoxy from ahearing to the wood as well as if it is unsealed, but I'm not looking for ultimate strength here, in most areas the cloth is more for damage resistance than it is for anything else. At the same time I don't want the epoxy peeling off in sheets because it won't stick to the wood well enough. I was thinking that.since the sealer still lets the stain get to the wood, it isn't going to be as bad as a coating under the epoxy, but I understand it won't be as strong as the bond I get with epoxy on stripped and sanded wood. The question is, will the bond I get with a sealer still be as strong as the wood, or is it going to be a lot weaker? If it is still stronger than the wood, then it's probably good enough.

I'm also going to try weting the wood prior to the staining, to help the grain wick up water instead of stain, but not sure if that will help enough.

Perhaps our friend Par has some experience here and can suggest a course of action.

 

Just use a stain, not a stain sealer. The sealers tend to block the "pores" of the wood and this doesn't let epoxy get a good mechanical bond. If you use just a stain, some pores will be blocked, but the peel strength of the epoxy will only see as much as a 5% drop, which most can live with. I don't have data on typical stain/sealers peel strength reduction, but it's considerably worse.

Water bases dies are the least color fast, though tend to be more "brilliant". If you pre-wet the wood, you'll raise the grain, which will cause other issues. Generally I like to use pigments in the resin, before adding hardener and apply the "tinted" epoxy directly to the wood. You get the best of both worlds, a strong mechanical bond with the wood fibers and the color. I use powered acrylic pigments which can be found at a real paint store. Naturally, you'll have to play with the color combination, but unlike stains, once you get the mix, it doesn't change color as it dries. So, mix up the combination you like, apply it and it'll cure just as it appears when it was wet.

 

Par,

Thanks for you reply. Yes, the dye is more "brilliant" than the previous finish and I was looking for something that was a bit more subtle. I will hit a real paint store and try some pigments. I also might try to use some sealer on a test piece and then epoxy a cloth strip over it and see how bad the strength is.

 

Make a "base line" sample of raw wood with the same sheathing, so you have something to compare against.

 

Good idea. I'll do that

I was thinking that if it pulls the wood apart rather than peels off it's probably good enough though. If the bond to the wood is stronger than the wood it's strong enough. If it fails in the bond joint it's not worth even trying.

 

A better test is to skip the cloth and just apply a few coats of epoxy on each sample. When cured for a week (or more) scratch a cross hatch pattern in the coatings with a razor, knowing your depth is through the epoxy film. Next apply the stickiest tape you can find to the scratched coating and pull. This will test the peel strength of the two. Without a load cell, you can't tell how much difference there will be between them, but you should get an idea of how much better one is over the other. This eliminates any laminating differences if a cloth is applied.

 

a few comments from the 'epoxy side' - if applying epoxy over stain, always give the stain a lot of time to fully dry, especially oil based stains.

a clever way to get similar results is to 'tint' your epoxy or your varnish and skip the stain entirely

paul

 

I was going to stick to waterbased coloring, either stain or dyes.

I've heard too many horror stories with epoxy not sticking when applied over an oil based stain. Maybe if you dried it with a heat gun or something, but I wouldn't expect epoxy to stick to any surface with oil on it, wood included...

 

Epoxy will stick to both alkyd and water based stains and have about the same peel strength afterward too (amazingly enough), as mentioned, assuming they are good and dry. I mentioned tinting the goo previously, which is a technique I use to match things.

 

PAR is right (again!) - the 'trick' with the stains is letting them really dry (maybe a week or so, certainly more than 24 or 48 hours).

using brown and/or tile red pigments i can mix up all sorts of 'tinted' epoxy or varnish. I even carry (sell) - a 'warm tone dye' for varnish and the like that gives it a bit more color

paul

 

Since figuring out how to tint epoxy, I've used this technique to "sneak up" on matching repairs, with existing surfaces. Repairs to clear finishes are the most troublesome, so you have to find a way, of insuring you'll have a good color match, when everything dries. Stains tend to look considerably different under clear coats, compared to just on raw wood, making a match nearly impossible, unless you applied the original raw wood stain too. By tinting epoxy, you see what you get as soon as it goes on, so a few test samples to get the color, then apply. It's also very probable that bond/adhesion issues are solved this way as well as well. I use a standard acrylic dry pigment, available at any paint store.

 

I have also used liquid pigment/gels and thinned with solvent - using that as a 'stain' - generally about 1 part pigment to about 10 parts solvent...

paul