Vinyl sealer?

[multidon]

My finishing projects have always used either oil based wipe on varnishes (several acoustic instruments) or poly (which I used on a solid body electric). I am considering my first build using nitro lacquer and I have been studying its application first. I discovered that all the major brands of nitro lacquer recommend using something called a "vinyl sealer" first to achieve an even, level surface. Well, I have hearing for years that "plastic" finishes like poly are "tone killers" and that nitro is so much better for tone because it allows the wood to "breathe", while the plastic "seals" the wood, in a bad way. Question: Isn't vinyl plastic? What gives nitro superiority over "plastic" when you have to spray a coating of plastic over your instrument anyway before you spray the nitro?

[sunburst]

Well, I have hearing for years that "plastic" finishes like poly are "tone killers" and that nitro is so much better for tone because it allows the wood to "breathe", while the plastic "seals" the wood, in a bad way.

Ignore all that. If there is any truth to it at all, it is way overstated there, and a thin finish (desirable) minimizes the amount of finish material, and so minimizes any effect of the finish on the sound, so better, thinner finishes have less effect on sound than thicker, heavier finishes, if the effects of finish on sound can be detected at all.
Onward to vinyl sealer. I don't like it, but it's because of the way it looks rather than any detrimental sound issues. There is a slight general lack of clarity with a vinyl sealer that I don't like. I prefer to seal with shellac for nitro or for varnish.

[multidon]

Thank you John. I will leave vinyl to the house siding industry and seal my wood with fresh shellac. Sounds like a much better idea to me.

[Dale Ludewig]

There's a number of things going on here. Firstly, I have and have never had any clear idea of what people mean when they use the term "breathe". The purpose of any finish is to protect the wood from the elements, to various degrees. With the possible exception of modern catalyzed finish, the finish usually only slows down any penetration of stuff. Or at least some stuff. And in that a mandolin or guitar is usually unfinished on the inside, maybe it only has one lung working, but it's still breathing. In thicker wood, as in furniture, solid wood that is only finished on one side is a guarantee of warpage.
I think people may be confusing terms and the fact that most poly type finishes are applied much thicker than say, french polish, or lacquer, and that is an issue. The plates can't respond as well because the extra weight and stiffness added from a thick finish. Another issue is flexibility of the finish, and that's going to vary not just due to thickness but to the formulation of the specific product being used. Plasticizers are added to nitro lacquers, and I would assume (because I'm not as familiar with poly and other plastic type varnishes) that they also have additives to make them more flexible, hence responding better to vibration and to resist crazing during a rapid change from cold to warm.
As to the vinyl sealer, traditional nitro lacquers like those we use in finishing musical instruments don't adhere well to raw wood. There are exceptions: self-sealing lacquers, but those are mostly, to my knowledge, used in the general woodworking industry (I use them a lot.) So the vinyl sealer is that intermediate coat and it's common to use the specific vinyl sealer recommended for a particular nitro lacquer. Its leveling characteristics are dependent on the product. I usually only put on a couple of coats after bursting, then scrape the binding, then another coat of vinyl sealer. There's not enough on to make it a plastic coating. In fact, I don't even sand before starting top coats of lacquer. I only sand after several more coats of lacquer.
I was told many years ago by one of my salesmen from Star Chemical (whose products are now sold to some degree by Mohawk) that it would be better to finish a piece of furniture entirely with sealer than to finish it with lacquer and no sealer; such was the adhesion concerns.
So there.......

[Dale Ludewig]

See, I rambled on too much and John was more concise. We did it again.

[greg_tsam]

See, I rambled on too much and John was more concise. We did it again.

Au contraire, mon frère. I gathered kernels of knowledge from both posts. Don't forget there are uneducated lurkers, like me, reading so that when we finally get around to building we will have some knowledge to use.

[Primarilywoodcharlie]

As a long time furniture fixer and repairman I only use vinyl sealer when I am trying to put a lacquer ( or C.A B.) finish over something that might be contaminated by wax or crud or silicone. Vinyl sealer is good as a barrier to these pollutants so that the lacquer coats will not fish-eye or wrinkle.
In my opinion, if you are finishing new, you should use a little lacquer sanding sealer and sand that mostly off. Just use enough to staunch the grain raising problem in the maple, then use only the best lacquer in as few coats as possible to get the desired build.

[dunwell]

It is also useful as a seal coat between the colors and before a french polish if you put the colors on with a shellac base. This makes a lock coat that won't dissolve and mess the color coats when you start fadding on the shellac for the FP finish. But as noted, just a very light coat of the vinyl sealer is good enough.

Alan D.