How to fix some pin holes in lacquer

[Arron]

Hi. Some weeks ago, when my spraying skills were even more modest then they are now, I sprayed some objects with Wattyl stylwood (nitrocellulose lacquer). They ended up with a sprinking of air bubbles. The lacquer dried and the bubbles popped but they left tiny pin-hole craters in the surface. Now I dont know how to repair them. I dont want to sand all the lacquer off and start again because inevitably that will leave it looking worse then they are now (or will be, if I can repair them as-is). I tried pulling the lacquer, but the holes are too deep for that. Can anyone advise how to fix these.
cheers
Arron

[gyropilot]

Unless someone has a better fix, I would use 400 and sand. Remember to stir (slowly) not shake the stylwood to reduce any bubbles.....

Geoff

[Christos]

Now this is just going by memory but I thought you can just apply another coat of lacquer as that binds with the last coat.

[chambezio]

With nitrocellulose the next coat slightly melts the previous coat as it dries. From memory (When we sprayed timber doors when I was working) give the actual bubble affected area with some fine wet and dry or steel wool. You are rubbing down/flattening the "crater" because that is slightly higher than the surface around it. Wipe any dust away and re-coat. These bubbles can come from small cracks in the timber. When you apply lacquer the evaporation of the solvent cools the surface but where these cracks are they have ambient temperature air in them and the cooling of the surface makes that warmer air rise to the surface and exploding on your job. All this is in micro size happenings.

[wood artist]

Hi Arron,
If i had this problem i would sand with wet or dry fine about 400 grit , not just the pin holes but most of the surface of the work. I would then mist coats on leaving each coat to dry before applying the next . Be sure that you only mist each coat , this is to allow the solvent to dry before it softens the lacquer already on the job . You did not say if the entire surface was affected or only in places . In places i hope or you may have to strip the surface . If this does not work please contact me for other ideas as i have experience in french polishing and may have other ways around the problem .
cheers Barry

[Arron]

In this case the bulk of the hole is actually below the surface. What I was finding here is that subsequent coatings with lacquer tended to fill the bubble holes a bit, but not enough, so the hole shrank with each coat but you still end up with a dimple. Problem is I dont want too many coats of lacquer (avoiding the plasticy look).

Anyway, today I did some experiments.
1. filled the holes with lacquer, applied by the tip of a pin. Worked OK, except that I ended up with little mounds rather then holes. I could sand these off, but its a pain sanding stylwood and its destructive and I'd rather just not do it.
2. tried again with a mix of 3 parts thinner to 1 part lacquer. Much better, not much sanding required.
3. tried filling the holes with colour matched Timbermate, sanding then recoating. Not a bad result.
4. tried doing some pull over, spraying another coat and doing pullover after it, then another coat and more pullover. Then fine sanded with 1200. Then sprayed final coat. Looked good - like polished glass.

I guess what I really want to know is how do the professionals do it. Whats the standard industry solution to problems like these - rather then the mickey-mouse solutions that I come up with ? With my solutions, you get there in the end, but I"m sure there are easier ways.

Chambezio, your explanation about the holes sounds logical - so whats the solution ? Cool the workpiece first perhaps ????

******** edit ***** Barry, you posted while I was typing. I guess with your experience I can rely on your advice as being the industry solution. *******

cheers
Arron

[Arron]

Hi Arron,
If i had this problem i would sand with wet or dry fine about 400 grit , not just the pin holes but most of the surface of the work. I would then mist coats on leaving each coat to dry before applying the next . Be sure that you only mist each coat , this is to allow the solvent to dry before it softens the lacquer already on the job . You did not say if the entire surface was affected or only in places . In places i hope or you may have to strip the surface . If this does not work please contact me for other ideas as i have experience in french polishing and may have other ways around the problem .
cheers Barry

thanks Barry. I've read this but one bit I dont understand. Why do you say to mist the coats on - why is it important to prevent the solvent softening the surface thats already there ? I thought the beauty of lacquer was the way it softened the previous coat and merged with it ?

cheers again
Arron

[Sgian Dubh]

I guess what I really want to know is how do the professionals do it. Whats the standard industry solution to problems like these - rather then the mickey-mouse solutions that I come up with ? With my solutions, you get there in the end, but I"m sure there are easier ways. Arron

Pinholes tend to be more of a problem when spraying onto coarse textured timbers rather than fine grained. Typically, if I'm spraying lacquer on something coarse textured I use one of two approaches.

The first method is to spray a first spit or mist coat. Just enough polish to flow into the crevices and coat all the wood fibres. This seals the grain and locks in any air that can be trapped down in the crevices or wood. Subsequent coats can then be applied with confidence, but even here I usually apply a second coat a little thinner than the normal recommended 100 microns wet. A third coat should complete the job because beyond that the dry film can get too thick and you'll end up with cracking or Chinese writing. Pre-cat and post-cat lacquers are limited to a dry film thickness of about 100 microns to avoid this fault.

A second approach, one I seldom use, is to add a little lacquer retarder to the mix, up to about 5 or 7% retarder. You can apply a thicker first coat because the retarder slows down the cure meaning bubbles of air have time to work their way through the wet film before it skins over properly. I don't use this approach often because it means the film usually remains soft for too long for my liking, days even, which can lead to blocking and imprinting.

Generally, fine textured timbers cause few problems of this type due to trapped air leading to pinholes in the polish, and you should be able to apply all coats at a normal wet film thickness (of about 100 microns) as stated earlier. Slainte.

[Arron]

Thanks vry much. That all makes sense - I had a closer look and noticed that the bubbles mainly appear on veneer, which is more porous then solid timber because its somewhat overdried.
I'm trying the mist coats. Seems to work.
Cheers
Arron