Finish over finish

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Forum topic by Scott10 posted 04-14-2015 01:39 PM 611 views 0 times favorited 6 replies Add to Favorites Watch
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28 posts in 2459 days

04-14-2015 01:39 PM

I have some cabinet grade birch plywood that is prefinished on both sides. I would like to apply polyurethane or polycrylic over this finish.

Am I able to do this? If so, what should i do to prep the surface and what should I use as my topcoat?

Thanks guys

-- Scott

6 replies so far

View OSU55's profile


1039 posts in 1407 days

#1 posted 04-14-2015 02:02 PM

Do you know what the prefinish is? If compatible, scratching the prefinish and clearing the dust will do it. I would use scotchbrite for this, probably the gray. Best thing is to test compatibility on a spot/area that will be hidden. If it looks ok after a few hours dry time it should be good.

For solvent poly, wipe the scratched surface with MS. For WB polycrylic, wipe it with 50/50 DNA/water. If you are asking whether to use solvent poly or WB polycrylic – I wouldn’t put polycrylic on anything – very soft finish. If you want to use a WB poly, you might try Rustoleum Ultimate poly – about the best that comes from a bbs.

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28 posts in 2459 days

#2 posted 04-14-2015 02:07 PM

Unfortunately i don’t know whats under this finish. Solvent poly would be fine for my purposes, just didn’t know if it would bond to a sprayed lacquer finish or whatever else they may have used originally. I’ll try cleaning with ms then scuff sanding the back side before applying polyurethane.

If no problems in 24 hours I should be ok?

Thanks again

-- Scott

View pintodeluxe's profile


4824 posts in 2231 days

#3 posted 04-14-2015 06:43 PM

Why do you want to finish pre-finished plywood?
There are those that say any finish can be coated with any other finish as long as shellac is applied between. I say why? I say either accept the pre-finished plywood as it is, or start with premium hardwood ply that is unfinished. It will match your project better in the end.
Have you ever seen incompatible finishes crinkle as you spray them on? I assure you it is heartbreaking.
Maybe with more information I would be better able to answer your question.
Good luck with it.

-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush

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1039 posts in 1407 days

#4 posted 04-14-2015 07:41 PM

Scuff sand then clean with MS. 24 hrs should be plenty of time to show a problem. There is always the possibility of the poly flaking off a year or more down the road due to different expansion characteristics of different finishes as well as bonding. A fairly vigorous scratching of the pre-finish will help the bonding the most. I am curious why you want to clearcoat a clearcoat finish?

You can test for what the pre-finish is. Start with alcohol Apply a few drops of denatured alcohol to the furniture, as shown in the photo at left. Wait a few seconds; then touch the spot with a soft-bristle brush or a cloth. Shellac a popular finish before about 1920 will soften and turn a bit sticky. If it doesn’t, it’s not shellac. Move on to Step 2. 2. Try lacquer thinner Change your solvent to lacquer thinner, and repeat the first step by applying a few drops of it to a new spot on the surface. If after a couple of seconds the finish softens enough to almost flow, you have lacquer. (You already know it’s not shellac.) But if the finish only becomes tacky and you know that it was built in the last decade the finish could be water-based. To be sure, test further. 3. Maybe you need a stronger solvent Try touching a bit of xylene (available at hardware and paint stores) to a different part of the finish. If the test area gets gummy, you’re definitely looking at a water-based finish. What to do when nothing works If none of these solvents dissolves the old finish, it has to be one of the reactive finishes one that cures through a chemical reaction such as varnish or polyurethane. You must thoroughly sand (scuff up) these film finishes before you can apply another coat of finish.

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Fred Hargis

3839 posts in 1911 days

#5 posted 04-14-2015 08:37 PM

Most pre finished is a very tough UV cured, sometimes proprietary product. Columbia Forest products offers 4 versions, for example. I have painted pre finished plywood, and it worked out very well. The factory finish was so smooth that the paint went down just as smooth. Not sure why you would want to put another clear finish over it, but what i did to insure adhesion was to apply a coat of shellac, then top that with the paint. I’m sure it would work just as well with other finishes, the one caveat being if you use something “polyurethane”, be sure to use dewaxed shellac.

-- Our village hasn't lost it's idiot, he was elected to congress.

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28 posts in 2459 days

#6 posted 04-16-2015 01:49 PM

The background. The project is a pair of corn hole boxes for corn hole or “bean bags”. The plywood I got was given to me. Two sheets of 3/4” birch finished both sides from a local cabinet shop. The problem is the surface is very slick, so much so that the bean bags slide right down the front of the board. The bean bags are top of the line, dual sided and the boxes are built exactly to tournament standards.

I would like to put something over the clear coat that might make it a bit less slippery. Water based polyacrylic is what most pro boards have on them, supposedly it is just right as far as slickness of the board. I could paint them then clear coat over for protection also. I like the look of the wood and would like to keep it clear if possible. But I need to remedy this as easily as possible. 4 sets of boards have to be ready for a tournament in two weeks.

If I had to guess i’d assume that Fred Hargis is correct, its some UV hardened finish, maybe lacquer. I may test it tonight if I get home in time.

Thanks for all the help.

-- Scott

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