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water-based polyurethane finish over epoxy

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Forum topic by DCollison posted 01-22-2013 10:33 AM 4859 views 0 times favorited 5 replies Add to Favorites Watch
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DCollison

2 posts in 637 days


01-22-2013 10:33 AM

I have questions re: a finish on an unusual project.

I am affixing a paper collage to a wooden panel using a mixture of wheat paste and a heat applied adhesive (fusion 4000). I wish to apply a finish to the collage that I can rub out to a gloss finish.

The surface of the paper collage is not even. For instance, the edge of one paper may be higher than another at joints. I have applied two layers of West system epoxy over the collage surface to build up the surface so that it can be sanded without breaking through to the paper. Though the epoxy levels of its own accord, the surface isn’t perfectly even.

I sanded the epoxy surface with 400-grit wet/dry, and then applied a water-based polyurethane. However, I can see scratch lines on the epoxy from the wet/dry paper through the polyurethane finish.

Questions:

Do I need to sand epoxy before applying water-based polyurethane? Do I need to sand between coats of water-based polyurethane? In either instance, what grade(s) of sand paper should I use?

Is there a better finish (i.e. lacquer) for rubbing out to a gloss finish?


5 replies so far

View PeteMoss's profile

PeteMoss

207 posts in 2156 days


#1 posted 01-22-2013 01:32 PM

Take what I am about to say as assumptions, because that is all that they are. The epoxy and poly will not have a chemical bond to each other, so it would seem that you would need to sand the epoxy to give the poly something to grab onto. Likewise, you would need of sand between coats of poly for the same reason. You should be able to do that sanding with a pretty high grit, like 300. As to what finish is best to rub out, I have no idea really, but I would think that you could do that to the epoxy without the need for a topcoat. You could probably look through the info on west systems website to see what people do to make those fancy CrisCraft type boats look so good. Good luck.

-- "Never measure......cut as many times as necessary." - PeteMoss

View Lee Barker's profile

Lee Barker

2169 posts in 1536 days


#2 posted 01-22-2013 04:01 PM

DCollison: Think of the resin on the redwood burl coffee table: Nothing can beat that for gloss.

If I could wish away the WB, I would, and encourage you to finish the piece with table top epoxy resin.

I don’t know how one would get the WB off.

Might we see a pic of the piece? It might help…

In general, I agree with Pete about scarifying a surface to gain adhesion with the next coat. However, that can be a chemical bond if your timing is correct. Often solvent varnish labels will advise you to recoat in, say, 3 – 6 hours (without sanding); if it goes over that, you must wait say, 12 hours (so the finish can harden thoroughly) and then sand and recoat.

Kindly,

Lee

-- "...in his brain, which is as dry as the remainder biscuit after a voyage, he hath strange places cramm'd with observation, the which he vents in mangled forms." --Shakespeare, "As You Like It"

View CharlieM1958's profile

CharlieM1958

15706 posts in 2904 days


#3 posted 01-22-2013 05:42 PM

I make no guarantees, but this is what I would try if I were in your shoes: I’d sand the poly off and get back to just epoxy, then try wet sanding the epoxy itself through a succession of grits all the way to around 1200. The final step, to achieve the gloss level you want, would be rubbing out with automotive polishing compound.

Having no personal experience with the West epoxy, I’m not 100% sure how it will respond to this. But I agree with Pete that there should not be a need for any kind of top coat over it.

-- Charlie M. "Woodworking - patience = firewood"

View gfadvm's profile

gfadvm

11226 posts in 1376 days


#4 posted 01-23-2013 03:13 AM

I agree totally with Charlie M. I have sanded/buffed West System 3 epoxy to a gloss finish. Be careful not to overheat the epoxy if power buffing. That will create an unfixable mess (don’t ask how I learned this!)

-- " I'll try to be nicer, if you'll try to be smarter" gfadvm

View DCollison's profile

DCollison

2 posts in 637 days


#5 posted 01-23-2013 08:14 AM

Thanks for all the advice, guys. I’ve been reluctant to use the epoxy without a finish coat of some sorts. I’ve read that most clear epoxies are not UV stable, i.e. they will degrade over time if exposed to sunlight/UV rays. Thus, the desire for a UV stable water-based polyurethane over the epoxy.

However, I looked about on the internet this evening and located a product—EX-74—that may be UV stable. I’ve emailed the manufacturer—TAP Plastics—to determine 1) if the product is UV stable; and 2) if sanding is required between coats.

I’ll let everyone know what I learn….thanks again for all your helpful thoughts!

Daniel

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