Catalyzed Varnish over Nitrocellulose Lacquer

[Ed A Hall]

I've got a piano that has a Nitrocellulose Lacquer finish but there are some imperfections in the finish. I sprayed several coats of nitrocellulose lacquer but when I went to sand it the next day, the finish balled up on the sandpaper especially with the finer grits such as 400. I know I'm suppose to wait for 2-3 weeks before sanding but I don't want to wait that long so I was thinking of an alternative.

How about using conversion varnish over the nitrocellulose lacquer? Does anyone know if it'll work. If not, how about using shellac to fill in all the imperfections followed by the conversion varnish? Or and alternative could be to use vinyl conversion sealer followed by conversion varnish?

[Scott Holmes]

Your lacquer is not completely dry don't rush it.

The Conv varnsih may have a adverse affect on the lacquer. If it does I suspect you will spend more than a day or two fixing it... Patience

[Howard Acheson]

I'm confused. Spraying NC lacquer over NC lacquer should not require more than a couple of hours to dry. Lacquer dries by the evaporation of the lacquer thinner. No curing or long on-going chemical action occurs. Are you sure the original finish was NC lacquer.

That said, I agree with Scott. Stay away from conversion varnish unless you first remove all the prior finish and then use the manufacturer's recommended initial sealer.

[Harvey Pascoe]

I usually manage to sand lacquer applied 3 coats on one day after 2-3 days, depends on how thick you applied it. If you really laid it on reallythick, its going to take longer. But it takes a full month before you can polish it.

[Ed A Hall]

I usually manage to sand lacquer applied 3 coats on one day after 2-3 days, depends on how thick you applied it. If you really laid it on reallythick, its going to take longer. But it takes a full month before you can polish it.

Even after laying 2-3 coats and waiting for 3 days, it'll still ball up when using fine sandpaper. The balls literally melt into the lacquer. I'm wondering if the problem could be the following:
1.) The fact that I'm sanding it while it's 105 degrees outside and high humidity.
2.) The nitrocellulose lacquer I'm using -- Deft gloss
3.) The sandpaper I'm using -- Norton 3X
4.) Not waiting long enough

Would it help if I thinned the lacquer with acetone instead? I'm currently thinning it out at about 50% and using an Iwata spray gun.

[Scott Holmes]

Deft is one of the softer NC lacquers. i don't know why nor do I use it.

[Rich Engelhardt]

Would it help if I thinned the lacquer with acetone instead? I'm currently thinning it out at about 50% and using an Iwata spray gun.
Deft is a brushing lacquer & not meant to be thinned or sprayed (except in the aerosol can).

Try brushing a couple of full coats on some scrap and see if it behaves differently.

[Ed A Hall]

Deft is one of the softer NC lacquers. i don't know why nor do I use it.

Sounds like this is my problem. It's causing me all sorts of grief because it's just not drying hard enough to even sand. Could you recommend a harder NC lacquer or is there something I could add to the lacquer to make it harder?

[Ed A Hall]

Just following up. I was able to sand it down with coarse sandpaper (180 grit) however, the stuff sanded off in small clumps rather than fine dust. At least with the coarse sandpaper, the clumps didn't melt into the coat and cause additional grief. I then sprayed 3 heavy coats of Deft nitro and it surprisingly filled the scratches from the coarse sandpaper and came out like glass.

I also tried sanding another panel that had cured for 3 weeks using finer sandpaper and it just made clumps. It appears like this stuff may be too soft to sand reliably.

I need to find another brand of Nitro that I can buy off the shelf that's harder.

[Howard Acheson]

Ed, Whether you use Deft lacquer or some other brand of NC lacquer, you are going to have problems trying to spray at temperatures over 80-85 degrees and relative humidity over 80%. I strongly recommend you call the finish manufacturer and find out their application parameters. I have heard of a number of car finishers who have not been able to spray what with the long term high temperatures and humidity in Texas.

On another point, you mention that you sprayed on a thick coat. That's not the way to apply lacquer. It should be sprayed in multiple thin coats. With thick coats the outside layer dries very fast leaving the inside of the film still wet. The dried surface prevents the escape of the lacquer thinner preventing the finish from drying. That, and the temperature, may very well be the cause of your problems.

[Ed A Hall]

Ed, Whether you use Deft lacquer or some other brand of NC lacquer, you are going to have problems trying to spray at temperatures over 80-85 degrees and relative humidity over 80%. I strongly recommend you call the finish manufacturer and find out their application parameters. I have heard of a number of car finishers who have not been able to spray what with the long term high temperatures and humidity in Texas.

On another point, you mention that you sprayed on a thick coat. That's not the way to apply lacquer. It should be sprayed in multiple thin coats. With thick coats the outside layer dries very fast leaving the inside of the film still wet. The dried surface prevents the escape of the lacquer thinner preventing the finish from drying. That, and the temperature, may very well be the cause of your problems.

I did previously try spraying very thin coats followed by 1 hour of drying time between coats but it still did not dry hard enough to prevent from balling up even after waiting for 3 weeks. The thick coats I sprayed was an attempt to correct / fill in the coarse scratches from having to resort to coarse sandpaper. That said, the panels came out smooth like glass. I think that may be the trick on using this stuff. However, it takes some good spraying technique to pull this off.

[Henry Ambrose]

Deft calls for a fairly heavy coat and as you've found out it will spray just fine when done correctly. You might try a different sand paper and also using only fresh areas of paper and moving the sanding dust off the surface as you go. Don't just grind it in or you'll get the result you are getting now. Deft is very soft and sands nicely by hand. You'll ball it up with a power sander unless you have a super light touch. The humidity is not helping you at all.

But you know all this from direct experience now....
Trust it.

[Ed A Hall]

Update:
I sprayed the rest of the piano this morning while it was 80 degrees and 86% relative humidity. I sprayed 4 heavy coats and it came out almost like glass. This stuff fills and smooths over the small imperfections very well compared to the nitro I had used before. It'll fill over scratches from a 180 sandpaper just fine. However, it just does not get hard like some stuff I've used in the past.

What I've found the best for sanding this stuff is a random orbital sander with no finer than 180 grit. Any finer and it'll clog right up. Sanding by hand resulted in clogged sandpaper in just minutes. After sanding with 180 grit, I spray 3 more heavy coats and it comes out smooth like glass.

Anyways, I'm beginning to like this stuff but it sure is different than the nitro I used 10 years ago. I had to completely change my technique.

[Henry Ambrose]

It'll get harder in a month or so but its just not as hard as some other lacquers.
Glad to hear its working for you.