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This is my second build; Behlen (mohawk) SI Nitro Lacquer.

So I sprayed 15 coats, targeted it to dry for 6 weeks before buffing.
Week 5, I have a long crack on the spruce top. Hope you can see it in the pic. I can feel the crack (not on the top, just the finish) with fingernail. Crack does not follow the grain line either. Wondering what caused it, how to prevent it and how to fix it before finish sand and buff.

The same thing happened on my first build, several months after buffing.
I'm using the recommended Behlen schedule with a jamb gun, with Behlen standard nitro thinner and retarder: Three coats a day max.

Before wet sand and buff, I was going to either paint brush on a 25/75 lacquer/ thinner along the crack to see if it would "uncrack". Alternately, mask off crack and spray it with thinned nitro.
Any thoughts on how to best recover this?

What is really puzzling is what is causing this? For the sake of full disclosure, I put a green Damp-it humidifier in the body of this second build (was 35% Rh) for 3 days that slowly brought it up to 45%. Removing the dampit is when I first saw the crack. Probably coincidence, but my first build has the same type of long crack on the top.

Thanks for any insight.

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Measure Twice,

Karl Borum

Seems to me that it is a top crack, not just in finish, Nitro won't crack along a line like this by itself.
Probably caused by low humidity. The top crack will need to be addressed before finish can be repaired, imo.

put a mirror in so that you can see the crack area and wash with some naptha. Apply light pressure along the crack. If you see the naptha bleed through you have a top crack. I never have done this on a top, but I have done it on a back that had the same sort of finish cracks.

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http://www.Harvestmoonguitars.com

It looks like the crack stops at the rosette -- Is this correct? If so, you have all the information you need to diagnosis what happened.

If the above is correct, tell us what you think happened and we'll chime in with additional information and how you can prevent his from occurring in the future. And from what I can see - the build looks very clean and professional -- incredible job for a second guitar !!!!

I've had that happen twice. Both times the the lacquer was way too thick. One time it was brought on by some pretty severe temperature shifts as I moved the guitar from a friend's spray booth to my car in the middle of winter. The other was with a guitar that got dried out very quickly by the customer. Both times I stripped and resprayed and didn't have any further problems. Every spray gun is different, so use the application schedule as a guide. Ultimately you are trying to build to a target thickness, not a specific number of coats. My guess is that if you scrape the area for the fingerboard extension you will find that the lacquer is pretty thick.

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Tony Thatcher
Bozeman, Montana

For the latest and greatest repair tools for cracks, check out T J Tompson's stuff. I just repaired a BRW guitar with 3- 5" long cracks on the back with this tool. It worked really well. No reason it couldn't be used on a top crack too.

https://proluthiertools.com/product/back-crack-repair-kit/

Karl says it's not a crack in the wood and that it doesn't follow a grain line. If the crack was in the wood, wouldn't the upper face brace act as a crack stopper and keep it from getting that long?

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Hi Jay,
A brace running perpendicular to the grain will help somewhat but definitely won't prevent low RH cracking. A cross-grain patch would be more effective for that than a brace but when a top gets low enough RH to crack, the bracing won't do much to stop it.

From that photo, it looks to me like the crack stops exactly at the rosette. Given the season and how easy it is to drop the RH to a very low range, I'm guessing that it is just a humidity issue. With nitro, you can also have cold checking or if the finish was laid on very thick, I can also see that being an issue - but you should still be able to put 0.010" thick nitro finish and not have that issue. After all, I bet most factory sunbursts are at a least that thick.

A better picture and more clarification from the OP will help diagnosis the problem.

First, thanks all for taking the time to read and reply.

Toonces, the top is not cracked by all estimations. THe crack has now turned and followed the curve of the rosette, just off the rosette on the finish over the spruce. Its really hard to photograph but I did my best.



THanks for that trick John. Flooded the crack with naptha, over several applications and worked the top (it doesn't flex a mm across that area). Then Used the Erlywine suction cup trick to push it under some pressure. With the mirror, no wicking, no sign of naptha, I'm pretty sure now it is not a crack.

Toonces, the top is not cracked by all estimations. The crack has now turned and followed the curve of the rosette, just off the rosette on the spruce.

The vinyl sealer under the lacquer is supposed to be flexible enough to prevent humidity cracking, as is the "stringed instrument" My shop is in the insulated walkout basement, today 68 degrees, 38% RH by digital meter. The meter indicates 32% is lowest, which was probably just before I turned on/ serviced the WH humidifier last week.

I appreciate the help with this.

I'll call it experience (lack of), likely too thick lacquer. I'm going to chip off under the FB extension later today. My first guitar cracked in almost the same place, opposite side, and meanders down the side of the rosewood body.

Now to the fix, I have to address it before wet sanding right?
I am positively not going to strip it and re-lacquer. I am proposing three methods to reduce the crack, which at this point is cosmetic (awfully cosmetic):

1) Quickly with Artists brush, very thin reduced and retarded lacquer, painting the 1 to 9 (lacquer to reducer) along the crack with one or two quick strokes.

2) Spray Val: Mask top and spray this 1 to 9 with Spray Val

3) Whip tip: same 1 to 9 along the crack (whip tip on plastic pipette)

I realize it may crack more down the road, I'm willing to call that "relic-ing" and move on to the next build, learning from this one. This guitar has been a source of much learning.

Thanks for all the help. I will nail this down so it wont happen with next one.

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Measure Twice,

Karl Borum

Unbelievable, this crack has moved 7 more inches toward the lower bout.

I'm adding #4 to my list of possible remedies:

1) Quickly with Artists brush, very thin reduced and retarded lacquer, painting the 1 to 9 (lacquer to reducer) along the crack with one or two quick strokes.

2) Spray Val: Mask top and spray this 1 to 9 with Spray Val

3) Whip tip: same 1 to 9 along the crack (whip tip on plastic pipette)

4) Light misting with Blush Reducer to "relieve" the stress along the crack

Thoughts?

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Measure Twice,

Karl Borum

From experience. If you have one crack, you will get more. Strip it and respray. It's a lot easier now than when the bridge and neck are on. I know it's a hassle. But you will learn and each time you will get better.

Quick edit. Fix options might be to just spray with thinner and just a touch of nitro. Get that area to melt back together. I have not tried it myself.

My 2 cents.

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Tony Thatcher
Bozeman, Montana

Way back in the dark ages my high school woodshop teacher taught me to apply lacquer by applying one coat/sand one coat, repeat until all the shiny spots disappear. Been doing that ever since. Sanding it back and blowing on another fifteen coats may well give the same results all over again. I suggest doing something different.

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Peter Havriluk

I'm looking at that one pic of the rosette where the neck covers it...I'm thinking some of what I'm seeing is residual glue...still, I'm seeing what I consider to be an obscene amount of finish...what I'm also seeing is the first pic that shows the sides, and that looks awfully like a heavy layer of finish...e.g. I'm in the camp with others that said finish thickness has caused this problem.

I'd consider sanding back a lot of what's there before I tried to reamalgamate by either straight retarder or heavy retarder with some finish...

just my 2 pence

If the crack is actually following the rosette now - then I am inclined to agree with the finish being too thick. You will have some slight movement with humidity changes and the finish needs to be flexible enough to handle that. Either the type of finish you are using is not designed for top coating wood or you've applied it too heavily. Make sure you are using a nitrocellulose lacquer that is formulated for guitars.

Best of luck with the repair !!!

Just a closing note, the entire box had been sanded back, flat, twice, confirming the original finish had shrunken back to show the wood grain (Behlen Stringed Instrument Lacquer). Jeff Jewett, in his online finishing class, said that was likely if the finish was not allowed to dry for 6 weeks. Based on that, and the shrinking finish, I re-sprayed until the wood grain was no longer visible (2 wet coats, two thin flow coats) and set it aside to dry for 6 weeks. That second over spray would account for the abundance of finish in the rosette channel, which was never sanded flat. Finishing can sure make a newbie feel stupid.

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Measure Twice,

Karl Borum

What is it that you see in the first photo that tells you the finish is heavy?

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

I honestly can't find words per se to answer your question other than it just looks thick to me (and yeah, it's a lower quality picture that doesn't zoom in all that well)...what I can say is I've sprayed some furniture with PPG DCA468 (acrylic automotive lacquer with a LOT of plasticizer) with like 15 heavy coats on the top and it had that same type of look as far as the way light passes through it before it hits the wood (in the case of what I describe the finish lasted quite well until some idiot during a remodel of the bar put a damned rag soaked with goof off on it...it's only real "flaw" what that impacts and stuff would discolor the lacquer but it still bent and flowed with the damage and stayed intact as opposed to chipping and stuff)

I seriously doubt Behlen instrument lacquer has nearly as much plasticizer in it and hence knowing what thick coats of harder top coats can do (learned the hard way of course) I'm in the camp thinking it's too thick for what it's being used for (a thin soft piece of wood)

Well, that was an easy fix. Whip-tipped some very thin, retarded lacquer along the crack, 3X. We'll see if it comes back. Thanks all.

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Measure Twice,

Karl Borum

You might want to sand the finish a bit to thin it out, but I guess in your first post you did say you had that to do yet. Thinner finishes are more resistant to cracking but still do, especially this time of year when going from cold to hot.

Five years later I’ve returned to finish this post. Thinner to lacquer 9 to 1 amalgamation worked, for about a week. After the bass side was “repaired”, the treble side started a running crack. The bass side developed another crack in similar place. The ultimate fix was to scrape the top down, reseal and re-lacquer a thinner finish this time.

Thanks again for all the help. The finish was in fact VERY thick on the top.

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Measure Twice,

Karl Borum

I was thinking that 15 was a lot of coats…

Back to my earlier comment about blow one, scrape one - - - until no more shiny spots. We ain't painting cars.

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Peter Havriluk