Hairline cracks in new lacquer finish - created 10-23-2010

Zentz, Ron - 10/23/2010.14:45:00
Amateur Luthier

Has anyone experience localized cracks in the lacquer finish? A few cracks developed on the top close to the waist on each side shortly after I moved the quitar to the hard shell case and took it into the house from the work shop. I do have an "Oasis" case humidifier but stupid me, I realized later that the silica gel bags that came in the case were still in there. I'm wodering if low humidity caused the cracks when the wood contracted a bit. Whatever the cause, no new ones have come up. The finish is fine exept the thin hairline cracks and they are hard to see unless looking closely. Would applying some lacquer thinner or naptha to the crack with a micro-pipette work to "flow" the lacquer and seal the crack? Other suggestions? I may just leave them alone and chalk it up to experience. It's my first build after all.

What kind of lacquer did you use?

Well, I have not purchased a spray system yet, so I used the aerosol type this time. It worked pretty well but it took a lot coats to build the finish. It's fine on the sides and back (walnut) and neck (mahogany).

Ron, Barry meant which type of lacquer, eg: Nitrocellulose, Polyurethane, Polyester, Acrylic, Enamel, Water-Based, etc.

Ah, thanks. Nitrocellulose lacquer in aerosol spray cans. In reading the Stew-mac book on finishing with spray equipment, each layer essentially "flows" the previous layer when sprayed. And for repairing, its bonds to itself also. So this is a function of both the lacquer and the solvent. I wondered if just flowing a little of the solvent into the crack by capillary action would "melt" the edges of the crack so they would flow together, sealing the crack and perhaps making it disappear. Sounds to good to be true but maybe it would work. This is really the first time I've ever used nitrocellulose lacquer. For furniture projects I've used many other finishes.

Thanks Ron.

For nitrocellulose, lacquer thinner, butyl cellosolve or further application of finish followed by level sanding will all help with your problem. Judicious application is advised for all of the options.

By the way, naphtha does not dissolve nitro, & will not help here.

And what brand?

Perhaps you sprayed too thick of a finish? And thick finishes will eventually craze. Also, the spray cans seem to make a finish more prone to a cracking than using one of the musical instrument lacquers in a spray gun.

Running lacquer thinner over the crack may make the situation worse. The thinner will run over the surface and be hard to control. Wherever it goes, it will soften the lacquer and you will have to re-sand and buff the finish again. Also, the finish will still be prone to cracks as you haven't changed anything so you will likely get more. I would leave well enough alone.

I too have had more problems with aerosol finishes cracking versus using my spray gun.

Did you wetsand the finish? Is there a chance that some water seeped in somewhere to the bare wood? That is a certain way to produce a crack in the lacquer, and I don't know this because I'm smart.

Thanks everyone. I used Grizzly Tools' lacquer, which says it's for instruments. They sell some kits and lutherie supplies and I've bought some tools there in the past. I'll be buying a HVLP system sometime soon...maybe Santa will bring me one this year? I may give the crack repair a try but I have been showing the guitar to friends and playing it...no one has noticed the cracks. Since I don't plan to sell it and it sounds great I may leave it alone. I did wet sand it and when I leveled the lacquer the first time (implying I did it more than once, which I did), I had a small burn-through area and a crack. I think it was the problem you mentioned Terry. The second round though, I don't think it's too thin or thick but it is my first time, so I'm not certain. There is a local luthier near me who has been helpful and he is interested to see how this came out. He may have some advice as well.

Since I don't plan to sell it and it sounds great I may leave it alone.

I would. You're not going to get everything right the first time, accept it and move on. Think about revisiting the finish when you have more experience if it bothers you later.

Just because Grizzly says its for instruments does not make it a quality product, in fact I would suspect its a rebranded furniture lacquer which usually has little plasticizers added to the mix.

If the crack had been caused by moisture infiltration you would see swelling and other signs, not just a single crack. If the finish is not too thick and you have a single crack, then likely the finish is not plastic enough to stretch along with normal wood expansion. This is fairly common when applying a furniture lacquer onto very thin wood that is only finished on one side.

Get a better grade of lacquer next time.

I'm pretty sure the Grizzly Instrument lacquer is actually Behlens. I have zero experience with rattle cans, so that may still be the problem. I have used Mohawk/Behlens instrument lacquer with good results though. With pretty much any nitro anything over 4 mils is too thick.

Even some of the best lacquers will check and craze over time. It is very often caused by rapid temperature and humidity changes. Most of the old lacquered instruments have it to some degree. You just got a "vintage"finish a bit early.

My son had that happen to a guitar I gave him. He was upset until I told him some people work to get a finish like that and rub a little dirt into the cracks so they show up better!

Thanks everyone for your comments and suggestions. Clay, we have a lot of good, black dirt here in NW Indiana...sounds like a plan!

Woodrow, I'm assuming 4 mils is 4 thousanths of an inch...if not please let me know. My geometry teacher in high school was a military guy and he used mils to refer to an angular measurement. In my work I'm more used to mm and microns, etc.

You are correct. 1 mil = .001" 1 mil = 25.4 microns