** ANSWERED ** A Question About Spray Lacquering

BillDL

The question is:

Is there such a thing as leaving the interval between successive coats of nitrocellulose spray lacquer too long?

Let me explain why I'm asking.

I am in the middle of spray finishing two electric guitar bodies with aerosol cans of nitrocellulose lacquer.  I had a slight setback in that while I was spraying the metallic and tinted colour coats in the garage of my house with the door open on a reasonably warm and perfectly dry day, giant black clouds rolled in very quickly almost out of nowhere and I could actually feel the humidity in the air increasing.  I stupidly decided to give them another quick coat and then leave them hanging to dry, and the lacquer began to bloom in a couple of places.  I stopped, cursed myself, and have allowed the bodies to dry for 3 days (2 of those indoors after the fumes had dissipated where it's warmer), during which the bloom has thankfully rectified itself.

The weather has now become crappy, and there has been drizzle and rain now for several days in a row, with the warm air so filled with humidity I can feel the oppressive thickness, so I have had to set the bodies aside indoors and will need to wait until dry weather returns to finish them.

In the past the longest I've usually left successive lacquer coats is probably 2 days at most.  In normal circumstances on a moderately warm and dry day I have sprayed on subsequent coats after leaving the piece for an hour or two until the lacquer feels dry and the fumes have lessened and have sometimes been able to finish a job in a full day, but maybe leaving the last coat or two of clear until the next day to see if I need fine sanding.  Obviously the beauty of nitro lacquer is that it melts into the upper layer of the previous coat and sometimes even self-levels slight irregularities without the need for sanding.  I also realise that applying the next coat of lacquer while the previous one is still soft can cause runs and a thick and soft build up that takes longer to gas off and dry.

The question is, if this crap damp weather persists for weeks on end and the lacquer on the bodies completely dries to a hard shell with no more fumes to gas off, will fresh lacquer sprayed onto that fully hardened lacquer still be able to melt into the last layer as effectively to help bond it, or is it more likely that I can end up with what almost amounts to a paper thin separate layer of lacquer on top of the previous hard layer as you would get if you just tried to spray nitro onto a poly surface without priming?

Winny_Pooh

The beauty of nitro is that it Does melt in, even after weeks, months, years. Just as long as you don't contaminate the current finish with oil or silicone you'll be fine. 

GoldenEraGuitars

Cellulose lacquers use chemical adhesion as opposed to mechanical adhesion (Think along the lines of 2 pack acrylic lacquer). 

With chemical adhesion the next coat will melt and adhere to the previous coat assuming it is the same product.

Mechanical adhesion is by means of sanding or keying the surface of the previous coat. This gives the new coat something to grip.

If you are in any doubt about spraying your nitro and how it will melt into the previous coats, you can spray a flash coat on to the body which softens the previous layer. This will promote adhesion for the next coat of lacquer you spray. You shouldn’t need to do this, though. And “flash coat” shouldn’t be mistaken with the marketing nonsense that fender comes out with… A flash coat is not a type of lacquer. It is a mixture of lacquer with a high concentration of thinner.

BillDL

Thank you gents.  That fully answers my concerns.  Although the thinned flash coat is a good suggestion, and is what I used to do with certain paints while spraying mechanical components and machines years ago, I would have no means of applying thinned lacquer as I am using aerosol cans.