Fixing badly applied lacquer with polyurethane.

[Cesar Vega]

I have a piece of furniture to which finish I have to fix.
It had opaque lacquer paint.

It has horrible brush marks all over the place.

Using a left over sample with the same finish as this furniture, I tried a layer of polyurethane (Minwax Oil Base Stain) on it. Beforehand, I sanded very lightly with 220 sandpaper, dry, cleaned the dust with a rag with some mineral spirits. I applied one hand.

Well, the resulting finish of polyurethane looked great, but it has some bubbles and dust.

So, to get rid of these details, I tried to fix this by sanding slightly with 600 grit sand paper, using mineral oil for lubrication. I have done this before at other works, and worked fine. But this time, the result was a finish that resembles the horrible lacquer finish it had, with grooves all over the place.

I have no idea what happened. It looks like the polyurethane adhered to the lacquer without really filling the brush marks, but adhering a thing layer on the pikes and valleys left by the lacquer. So, what it's left after the sanding is again, the previous grooves set up with the lacquer finish.

What would be a proper way to do this? Am I skipping a step?

Oh, btw, I also tried sanding the brush marks with #220 grit, but at some parts I sanded so much that the lacquer was removed, leaving the bare wood exposed. And anyway, the sample with the sanded lacquer, looked the same at the end of the same process of applying polyurethane then sanding with #600 and mineral oil.

What else can I do?

[John Keeton]

Cesar, I am no finishing guru, but I think you have diagnosed the problem correctly - "It looks like the polyurethane adhered to the lacquer without really filling the brush marks, but adhering a thing layer on the pikes and valleys left by the lacquer. So, what it's left after the sanding is again, the previous grooves set up with the lacquer finish."

That will be the case with any finish applied over another finish. In addition, I think you may have adherence issues with the poly over lacquer.

I am not aware of any lacquer paint that can be applied very effectively with a brush. It seems that sanding the lacquer finish smooth (or removing with lacquer thinner), and reapplying the same lacquer paint as a sprayed finish would be the answer.

[Bill Davis]

They do make lacquers for brushing by adding retarders so it does not dry so fast. Since lacquer is really meant to spray because of it's very fast drying solvents brushing is a big challenge. I've done this to make a successful brushed lacquer finish however. Applied enough coats with light sanding between each to build a substantially heavy layer. Then do what you did unsuccessfully to the poly, wet sand it to remove the unevenness caused by the brushing. It is important to lay on a heavy enough build of the lacquer to allow enough sanding to remove ridges without sanding through. All that can be a lot of work and quite unpleasant at that if you have other than nice flat surfaces on your piece. Best solution IMO is to spray lacquer. That doesn't automatically make a perfect finish but sure goes a long way toward making it easy to achieve. Once I sprayed lacquer I was finished brushing it. Never again.

[Joe Chritz]

If it is a pigmented lacquer coat you may be farther ahead to just refinish.

Sand down until flat and smooth and recoat with another pigmented top coat.

I really like the new formulation of Pro-Classic from Sherwin Williams for a paint. I assume you don't spray, if you do there are a lot of other options. Although I tend to stay with the Pro-Classic for paint projects.

You have the problem ID'd correctly I would say. If the finish isn't flat to begin with it really won't be after the finish. Not without some serious filling work.

Joe

[Howard Acheson]

What has happened is that your sanding of the poly varnish has removed the varnish from the top of the "hills" left from the brush marks. If you only applied a single coat of poly this would be expected as you did not build a thick enough film of the poly. If you plan to sand a finish, you should have 3-4 coats of the finish applied.

At this point, you can let the finishes fully dry for 4-5 days and then sand it flat with 320 paper on a flat rubber or felt covered sanding block. When you have the surface flat, apply a coat of your poly varnish. Let it dry a couple of days, flat sand again with 320 paper and apply two more coats. That should give you a nice finish.

[Cesar Vega]

Hi, Howard:

Thanks for your answer.
Yes, that's what happened, it seems.

My problem this is pigmented lacquer, 100% opaque, espresso/chocolate color, that I didn't paint, btw. A very very mate finish, zero bright.
I was told it was originally spray painted by one guy, then another different guy did a second coat by hand, the one that messed up the finish.

But it didn't ended very thick anyway. So, when I sanded it down in the samples, I got to the raw wood very easy.

So, if I sand more, I would end having to go to the painter shop with the spray gun since I don't have one.

This is what I was expecting:
I planned to put polyurethane on it, in the hope that it would sort of camouflage the brush marks. I know the brush marks will be somehow visible through the poly finish anyway.
As I told, applying the poly over the prior finished looked good enough already since it disguised the brush marks a lot.
The problem then was that the poly ended with small bubbles and dust.
Or may be I will just have to go and give several coats as advised, and then I can rub it out.












What has happened is that your sanding of the poly varnish has removed the varnish from the top of the "hills" left from the brush marks. If you only applied a single coat of poly this would be expected as you did not build a thick enough film of the poly. If you plan to sand a finish, you should have 3-4 coats of the finish applied.

At this point, you can let the finishes fully dry for 4-5 days and then sand it flat with 320 paper on a flat rubber or felt covered sanding block. When you have the surface flat, apply a coat of your poly varnish. Let it dry a couple of days, flat sand again with 320 paper and apply two more coats. That should give you a nice finish.

[Howard Acheson]

Unless the lacquer was specifically formulated to be a "brushing lacquer", brush will not give good results. Lacquer dries too fast to flow out and brush marks will almost always result. Even so called "brushing lacquer" can be problematic. Lacquer is best sprayed.

There are a couple of potential problems over coating lacquer with an oil based poly varnish. First, poly varnish is quite amber in color and will change the color of any light colored finish it's applied over. Second, the get good adhesion, the under coat must be scuff sanded with 320 sandpaper. If the under coat is not thick enough, there is a great chance of sand through to the wood. The is always a potential for problems when you mix finishes. It should be avoided in most cases.