Official Luthiers Forum!

if you don't need to reduce the product to spray, then I guess stirring is up for grabs...I always reduce and that of course requires fastidious stirring to assure proper dissolution of the solvent...spraying too many coats in a day is risky as if you don't allow for off gassing issues may arise (like little tiny bubbles in the finish) as you lay down subsequent coats.

when spraying heavy coats of lacquer on tables and such I spray a maximum of 5 coats in an 8 hour period and allow 3 days minimum before I add any more, and increase that time to 5 days if I used any retarder...on a spray day even fine sandpaper of 600 grit is OK between coats to knock off any trash, but I tend to use 220 on a new day's first coat as I feel the deeper scratches help the new layer burn in better...remember, towards the end of a build up of lacquer you are activating all of the lacquer with the solvents and allowing a bit more time between coats is a good idea to allow for solvents to off gas

I was wondering about subsequent coats softing the earlier coats with nitro. I kind of figured it might. When I use CA I know new material does soften earlier layers, even if they have been put down days before.

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Joe Beaver
Maker of Sawdust

It does soften some. This is another reason why it's best to wait a month after the last sprayed coat before you start leveling and buffing.

Regarding the original question about how many coats you can put on in a day: there is this very bad thing that can happen if you build up too much too soon. You get the job done and it is beautiful, but there is still some solvent lurking deep down. When it evaporates, the film shrinks, and suddenly you see sanding scratches that had been filled perfectly by the nitro before it shrank. This happens a couple of weeks after the customer has the guitar.
When I'm trying to push the number of coats: I hang it in the sun, turning often and never if it is hotter that 80 degrees F (the finish can bubble). I don't do another coat until I can put my nose right on the finish and not smell solvent.
I would never try this myself, but back when nitro was the only kind of lacquer Gibson would have the sealing step perfectly smooth then spray it with hot lacquer with very little solvent. As the thick coat of lacquer dryed there were no scratches underneath to show through. Saved money on solvent too. Hot spraying equipment was a furniture industry standard item.

Leonard,
What you are saying makes sense. I think I'll use the sniff test to set my schedule.

One thing I'm not sure about is the use of retarder to slow it down during humid conditions. I get the part about giving it more time to lose the moisture but how humid is humid? I'll be spraying outside and drying in an open garage so no climate control. At what relative humidity should I be concerned?

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Joe Beaver
Maker of Sawdust

Hi Joe
When you have uncontrolled climate there's always things that can and probably will go
wrong but there are a few do's and don'ts to avoid a sure bad outcome. IMO here's the most
important. Humidity, make sure your guitar and nitro, or any spray material, is warmer than the
ambient temp that you will spray in, ie don't bring guitar from cool garage to outdoors and spray on a muggy day and expect a good outcome. Very basic and simple but easy to overlook.
Hope this is helpful.
Ken

Ken,
Yes, very helpful. I live in pretty dry climate for the most part. Even then, when I use water based I wait for dry, warm weather. Maybe I should do the same with nitro.

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Joe Beaver
Maker of Sawdust