Finish checking/cracking

My friend, Greg, is too lo-tech to have a computer so I'm posting this for him.

When he sprays nitrocellulose on a new guitar he sometimes has a problem with checking (see attached picture). He builds up to 24 coats with wet sanding between. It looks great for a day or two after he's done then he gets checking on the top, spruce only. The back and sides have no problems regardless of RW or mahogany. The last time he sprayed and this happened it was in the wintertime. After some research I thought it might be from either 1. too dry in wintertime, 2. too many layers, 3 layers too thick on the spruce. Any help or suggestions would be greatly appreciated.

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Permalink Reply by Frank Ford on July 4, 2013 at 7:10pm

More info needed.

1. Exact lacquer being used - brand, etc. Some lacquers are not formulated for best results in our craft.

2. Precisely what constitutes "one coat." For example, when I spray lacquer, I go over the surface three times as I'm shooting. Is that "one coat?" After all, it's applied all at once. Is that three coats? Sanding between coats? I hope nobody sprays 24 separate coats and sands between each - talk about extra work!

3. How thick is it when it's dry? Total film thickness is critical to the issue of checking. Most of us (I think) aim for something in the range of .006" maximum. The thicker the lacquer the quicker the checking. It's easy to measure precise thickness - just apply some masking tape in the bridge area, or another place where finish will be removed. After it's dry and ready to buff, chip off and measure some flakes of finish.

By the way, the number of coats is a immaterial - it doesn't matter how the lacquer gets there - it's all about the final thickness.

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Permalink Reply by Terry Schafer on July 4, 2013 at 7:52pm

Thanks for the reply, Frank.

I didn't realize but the last time he used StewMac Professional Guitar Lacquer aerosol cans . (he has used compress but can't remember if it happened then).

He sprays once over, waits an hour or two, sprays once over, until he has about 12. let's it dry, wets sands at that point then puts 12 more on and wet sands the final. He sprays the same amount on back, sides and top.

He will have to try to measure the way you suggested. I don't think he's ever measured the thickness.

Should he use less on top than on sides and back, thinner on top?

He said he was using lacquer for pore filler/sealer then sanding it off. Should he use an acctual sealer?

How long should he wait for each coat before spraying on more?

Terry