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Why, when spraying finish, do we spray 3 coats, then sand, spray 3, sand.... Why not spray more than that?

Also, why must we wait 2 hours(ish) to spray another coat? EM6000 says to wait 20 minutes....

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Use whatever schedule works best for you to get a level finish. I like to level after 2nd coat, then spray the 3rd. (One hour beteen coats) Allow to cure overnight then sand level. 3 more coats with leveling in between as needed. Drop fill as needed , then cure for 4 weeks before final leveling and buffing. This is for shooting nitro BTW.

Chuck

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Well, I've found the first coats look like heck,
so, if you keep piling heck onto heck,
it don't get better.
I like to do 3 coats, level, 3 coats, etc.
Works nicely for me.

My experience with nitro is that sanding every three coats works well.

Also, drop fills are best done as early as possible in the process.

Mark

For me it's a cleanliness problem. I have a covered area outside that i spry in and can't keep things clean enough for more than a coulple of coats.

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Time between coats, as well as number of coats in a day/between sanding depends on the finish used. If you re coat too quick it can trap solvent underneath the upper coats. This causes bubbles and pin holes in the finish, often not to be seen until level sanding. Most manufacturers recomendations on time between coats is based on prefect drying conditions.

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Cue Tevya singing "Tradition"!

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"Act your age, not your shoe size" - Prince

With every finish you have to take into account shrinkage, which is different for every finish, even different brands of the same type of finish.
Evaporative finishes shrink a lot in the first 24 hours, and continue to shrink afterward. If you spray a glob of finish, first as Woody points out, the solvents will be much slower to evaporate (the finish may remain soft for a long period of time), second your levelling job before the last coats will be useless as everything will shrink back, and most probably not evenly. That means sand through before buffing…
There is no free lunch.
It seems some newer waterborne finishes are faster to cure and shrink, and that may be attractive along the more benign nature of the chemicals. Will they be viable in the long run?
Otherwise catalysed finishes (poly, urethane) are the way to go if you're willing to spend the $$$ in safety equipment, which you will need if you want to remain healthy, and probably alive (I would add that nitro lacquer is not the healthiest finish to work with…).
When using FP, lacquer or oil varnish it seems to me one should embrace the methods, as work intensive they may be, as it's the only way to get results.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

I spray about 20 coats and then sand all at the end - too afraid of burn through to do otherwise.

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Does anyone have any data on this or is this just the way we've always done things?

I do a lot of sun bursts and a sand through is catastrophic. I'd be afraid that the post 3 to 6 coat level sand would easily burn through (and I've done it which is why I spray all coats before any leveling). According to measurements I've taken during the finishing process, the film is only about .001 to .002 thick at that point (in a waterborne) and still very soft, often clogging the sandpaper very quickly. I don't know for sure, but I'm thinking that stearated sandpaper could be kind of risky in a waterborne finish with more coats to come.

It does stand to reason that allowing more time between coats will allow more of the volatiles to evaporate but, the manufacturer's advice is pretty contrary to the typical luthiery advice.

Convince me.

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-Which waterborne finish are you using, and what schedule is the formulator
recommending?

I use EM6000. Manufacturer suggests spraying coats like 20 minutes apart but, they use a much thinner final coat than we do.

Please remember folks that the OP mentioned EM6000, not nitro. I've never sprayed nitro (well, once out of a rattle can) but I understand it goes on much much thicker than EM6000. I find that I need to spray 20 coats to get an un-leveled thickness of around .008.

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