Grain Sinking with Nitrocellulose Finish on a Guitar Body

An old-fashioned nitro finish on a guitar can require multiple coats and many weeks of curing time. January 29, 2014

Question (WOODWEB Member) :
I have a millwork shop but normally don't do any finishing so I am somewhat inexperienced. I have built a few guitar bodies out of African mahogany and can't figure out why the grain keeps sinking. I filled the raw body with Daly's wood filler (oil based) and allowed several days of drying.

After two coats of sanding sealer and several days of dry time before sanding with 320 grit there are no low spots visible and it looked glass smooth. I then shot it with gloss nitro lacquer, about four or five coats and the finish was super smooth, with the intent of letting that cure for a couple weeks before a final sand and another couple finish coats. After about a week the grain in the mahogany has become so pronounced I am not sure I can sand them out without removing the top coat! I used an HVLP gravity air setup and Lemar lacquers. Any thoughts on why?

Forum Responses
(Finishing Forum)
From contributor S:
If you insist on using nitro may I suggest this system: base color; washcoat - sealer cut up to 100%; this seals the bottom and inside edges of the pore; pore filler with adequate dry time; pore filler again with adequate dry time; sealer; full scuff; top coat(s). With a higher solids post catalyzed coating system you may not need a synthetic pore filler, unless you desire a contrast between the base color and the pore coloring.

Also, when removing the pore filler allow it to flash a wee bit prior to removal; and remove up to 90% by working in a circular motion as this assists in packing the filler into the pore. Finish the wiping with the grain. Next day if there are any traces of filler left you can safely remove with a rag slightly damp with naptha.

From the original questioner:
Thanks for the reply. Is there any value to using shellac for the wash coat? I do like the amber tone the grain filler left and also that amber shellac leaves without actually staining. Lastly how about the body I have already sprayed, will it eventually stabilize?

From contributor S:

While I have never used shellac as a washcoat - and I have as a base coat for the reason you mentioned, I do not foresee a problem as you are using nitro as your coating. Meaning that balance between coatings and adherence shouldn't be an issue. However, it might be prudent to do a full set of samples not only to discern proper reduction for the shellac washcoat (try 50%) - too viscous and you are not leaving enough room or space for the filler to do its thing, and to insure proper adhesion - cross hatch, etc. It is a guitar after all.

As to your piece in progress, you could sand it back to wood while leaving the pores with finish and filler (there is a long and varied history of this type of pore filling for leveling purposes going back more than a bit up to at least mid to late 20th century ). Use shellac as the sealer and start the nitro top coats, or cut it back and keep applying relatively thin nitro top coats. If you choose the latter, wet sand 400 at least between coats. You will get there eventually. Be grateful it is not a dining table or a run for a hotel or some such thing.

From contributor J:
I have done a few guitars with nitro. I now use Conversion varnish because I am too lazy to do the nitro. You need to give the nitro about a month or two before you cut it back. It takes that long to fully cure, and if itís not fully cure the grain can sink back. Itís been a while, but I think I was around 15 to 20 coats. First five sprayed about ten minutes apart, let cure for a week then cut back. Repeat this one or two more times, let cure for a month, then repeat with another five coats and let cure for a month or two before buffing. None of the big guys use a true nitro anymore. A Fender custom shop has a couple coats of nitro over top of a modern day sealer. They just donít have the time it takes.

Go look at a brand new Gibson Les Paul custom shop, still a little of the grain telegraphing as they donít let it cure long enough. Both my CS Les Paulís do this. I have an R8 with VOS finish, and another custom/custom R8. In other words, have lots of patience, or just live with a little telegraphing. If you want to speed things up apply two coats of conversion varnish, wait a week and then cut it back smooth. Then go over top with your nitro. This will save about a month.

From the original questioner:
Thanks for the replies. The thought process behind nitro was twofold, keeping with a traditional vintage finish and something that would be not too technical to spray. I had done some test pieces on bubinga but evidently the mahogany pores are just that much deeper. I'll let this mahogany body sit and cure then sand back. Meanwhile Iíll start a couple others I have that are bubinga.

From contributor G:
ML Campbell makes a product called Level Sealer that you can spray on really heavy that sands great and speeds up the process considerably.