Recoating Over a Mystery Finish

We're not sure what's on this table, but it might be polyurethane. Can anything safely go over it? Now that's a messy subject. October 14, 2006

I have a customer who wants a table sprayed with finish to smooth out the finish. I think it was brushed with polyurethane, as it looks thick and the pores were not sealed. It looks like a form of mahogany. I propose to sand it down as much as possible without getting into white wood, then spray vinyl sealer and top off with lacquer (nitrocellulose). I am trying to avoid re-finishing. Will this work? Will this combination be compatible with polyurethane, and or other finishes if it is not polyurethane?

Forum Responses
(Finishing Forum)
From contributor A:
Why not try sanding it smooth, and then either rubbing it out with pumice, then polishing it up with rottenstone. You could also try sanding with 600 sandpaper up to 2000, and then compounding out the tabletop.

From contributor B:
I believe the lacquer will eat up the urethane quickly. The result will be lots of alligatoring, etc. and it will be worse. Usually with a table you lay it on thicker as it will not run but that will eat the poly up quickly. I've done it with thin, fast drying coats and sanding the alligatoring out but thatís not the way to go. I'd lay down plenty of stripper and wipe down, then spray. You'll save time and energy. If anyone has a good way to put a clear barrier coat first please let me know. I just remembered the thing to do is apply shellac as a barrier coat. I believe that will do the trick.

From contributor A:
Shellac is an excellent barrier coat, but not in this case. If you don't want to strip it, then stay with the poly.

From contributor C:
I have sprayed lacquer directly over poly several times, however contributor B is right - if you are not very careful it will eat up the poly. Use lacquer only - no sealer. Sealer has a lot more solvent. When you put the lacquer on you need about four mist coats to equal your first normal film coat. Even then you still have to put on really thin coats or you will sand down to bare wood. I usually use shellac as a barrier coat but even then you still have to use thin coats.

From contributor A:
Thin coats, to me, mean very little solids and lots of thinners. Itís the solvents that cause the bad reaction of non compatibility. Itís a gamble trying to use other coatings to correct this kind of a problem.

From contributor B:
Why is the shellac a bad choice in this case? I use lacquer over shellac all the time, mostly on upholstered furniture.

From contributor A:
Lacquer and shellac are both evaporative coatings, and they are very compatible and will dissolve in each other. Polyurethane is a reactive coating - it will not re-dissolve once it is cured. Any coating that is applied on cured poly is a gamble, and in most cases will fail.
Naturally, the best way to go is to strip it, and refinish it. The original questioner is looking for the easy way out and I am sure that others have different ways of doing it. I gave my opinion on how I would do it. Itís not the right way, but it is done.

From contributor D:
One other thing to remember is that people have the tendency to put things like Pledge and the like on these tables and when you shoot that coat of lacquer on it, you will get a top that has the texture of the moon with fisheyes and craters all over the place. I have been down that road and now have a policy to take the exit before I get there.

From contributor A:
To contributor D: That's one of the reasons why, rather then shooting over the table, I would prefer to sand out the brush marks, and then rub out or compound up the top.

From contributor C:
To contributor A: Have you ever tried rubbing out poly? When you rub through one uneven coat into another to get a flat surface you see the transition between coats. Itís not like lacquer. Shellac will work with any coating so far that I have ever tried and I have used it numerous times on poly and varnish with absolutely no problems as it is alcohol based.

From contributor B:
To contributor C: How do you apply the shellac? Do you cut it or spray it? I know it dries quickly and is used over latex paints for sealing (Bin brand) I believe, and it doesn't melt the latex.

From contributor C:
To contributor B: You can do any of the above. You can buy it in pint to 5 gallon cans and in spray cans. It depends on what you are going to put it on and how much you are using. I used to do a lot of table top refinishing in times past when things were slow. I used it a lot over the top of coatings if I wasnít going to sand to bare wood and start over. I never once had any issues using it.

And contrary to contributor Aís idea of "less solids" and more thinner, itís just the opposite. Iíve done it enough times. The first few times I put on a good heavy coat and boom. Stick with several mist coats to build so that you can use regular coats. Try it on a piece of scrap and you will see what I mean.

From contributor A:
To contributor C: I have done it many, many times. It is called telegraphing and it occurs in all reactive coatings where a coating is cured and another coating is applied on top. The key is to know how much to sand, rub, and polish, just as we would do with post cat coatings like polyester, 2k polyurethane, and CV coatings etc. Shellac, is one way of trying to repair this problem. Other ways are using water base, vinyl sealer - even glue sizing will act as a barrier coat. All of these coatings are a gamble, and in most cases the expansion and contractions of the substrates will do them in over time.

From contributor A:
You all keep on mentioning Shellac, but it has to be "dewaxed" Shellac.

From contributor C:
To contributor A: Absolutely correct, it does. Zinsser in the cans, for primer purposes, is.

From the original questioner:
You guys are amazing. I can always count on a great discussion. I've been on vacation, but have got to get to work on the table. My gut feeling tells me to go sand it down and spray polyurethane over it. Then if my guess is correct (that it is indeed poly) I should have no problem.

But if it is not poly, will I be asking for more trouble? And, can I use a waterbase poly? As contributor A said, I am looking for the easy way out, and I do plan to test my plan on an extra table leaf.

From contributor E:
It looks like you've received some good advice but I would question some of it too. The fundamental issue of course is that the thinners in lacquer when applied over reactive or coalescing finishes act like a stripper; softening and swelling the original finish and leaving a mess. I've found it to be more of a problem with conventional varnishes than with polyurethanes but it is always a risk. Try rubbing some lacquer thinner in an inconspicuous place with a q-tip. If the finish dissolves, it's lacquer; if it softens and swells it's a varnish of some type and you don't want to spray lacquer over it. If nothing happens try straight acetone. If it softens the finish you may be able to get by with thin coats of lacquer; if nothing happens - spray away. Several "mist" or thin coats of lacquer (not the same as several coats of thinned lacquer) on less sensitive finishes works because the thinner will flash off before it has time to do any damage.

Dewaxed shellac will work as a barrier for lacquer because its solvent will not attack the original coating unless it too is shellac and it will stick fairly well to poly. The only potential problem is that dewaxed shellac can be pretty brittle and may create a problem if it's sandwiched between two flexible materials. I would be concerned with potential adhesion problems with any other material as a barrier coat.

Sanding will most likely reveal lines that are the interfaces between coats and I've never been able to rub them out. The only thing that's worked for me is to bury them with another coat of finish. No one has addressed the open pores. These can be filled with finish but that's a slow process. This is a case where gel stains work very well. Pick or mix a color that matches the existing finish; wipe it on, and scrub it off. Do this a couple times and you should get a good partial fill of the pores.

It's hard to make a specific recommendation without seeing the table but based on what you've described this is what I would try first:
1. Sand out the imperfections to 180 or even 220.
2. Partially fill the pores with gel satin.
3. Sand again w/220.
4. Topcoat. If you think the original coating is polyurethane you might as well stick with it. Just be aware you need a good mechanical bond between coats. I like to thin mine down and wipe it on but if you like to spray - go for it. If you want to use lacquer, start with a couple coats of Seal Coat and even then, keep the lacquer coats fairly thin (3 mil wet).