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This guitar body fell off the table onto hard cement floor impacting at the top corner in the lower bout region, cracking the lacquer on the top. Small crack lines fan slightly from the edge (tree like cracks) with a 7” crack along the top. It also whacked a spot of lacquer on the binding on the side.
New build, rosewood sides, mahogany bindings. This build is for me and not built to sell. It also has a Paua shell purfling on the top, so scraping everything off the top is reserved for PLAN B. I want to know if the lacquer thinner approach will work.
This type of lacquer damage/ repair is rather new to me (beginner in the craft). I’m going to document the repair; what worked and what didn’t. I’ve read the piece on the FRETS blog and am modifying Frank’s approach.

BACKGROUND:
• Mohawk Stringed Instrument Lacquer (SIL) over Mohawk vinyl sanding sealer over Sitka spruce top.
• Eight week cure on the fresh lacquer (was wet sanding when it happened, two weeks ago) .
• Entire top was sanded dead flat with P1000 when it happened.
• Top is not cracked, and braces are solid and attached.
• Crack is stable in that it hasn’t creeped or lengthened.

THE FIRST APPROACH (and hopefully it will work):
EDIT: ALL APPROACHES FAILED TO AFFECT A PERMINENT INVISABLE REPAIR- SCRAPING THE ENTIRE TOP NOW

Lacquer thickness in the area of the crack(s) has been thinned with sandpaper and careful target scraping with a half round scraper.

1. A mixture of 90% thinner/ 10% lacquer, 7% retarder (because it's between 5 and 10%) will be applied over the crack lines, with a fine line pipette. Hypothesis is that the thinned and retarded lacquer will wick into and along the crack and amalgamate the lacquer, eliminating the crack.

2. Dry overnight, sand with P320 (to see high and low spots. Mask an area ½ inch to either side of the crack region, then the same thinned lacquer will be lightly sprayed with a spot gun.

3. Thinned lacquer allowed to dry 5 minutes and a coat of 3:1 lacquer applied. Additional coats of 3:1 lacquer applied, an hour apart, three a day max, P400 “flat” sanded between coats, until level.

4. Drop fill as required. Same process will be extended around the edge, to the binding region of the impact on the binding.


5. Dry 7 days then resume the wet sanding process.

Im going to wait a day before I do this in case I get some really good input from the voices of experience on the forum. Will update as the repair progresses.

NOTE the cracks were barely visible until they were spot scraped, along their length, to reduce the thickness of the cracked lacquer....

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Measure Twice,

Karl Borum

George Grotz wrote a little book called "The Furniture Doctor" and mentioned a method of re-amalgamating the finish with lacquer thinner and rebrushing it to cure chips and cracks in the lacquer. It can be done but....
Since you are still in the middle of finishing I would be inclined to strip the top with lacquer thinner and respray it. I think the job would be more quickly done and would be easier to get a perfect result.

I have just done a repair like this
10% retarder to thinker use a thin brush and apply to crack. You can see it melt together, You will know as you sand and buff out it it held.
used this a number of time and it can work

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Interesting outcome.
[list=]First, the top actually cracked from the impact, two places, both the long crack on the left and the shatter crack on the right.

[*]Both top cracks stopped at the tone lower tone bar.

[*] Cracks are properly cleated now.

[*]I had shined light from the inside when I decided no light leakage meant the top was not cracked.

[*]I'll use naptha next time. [/list]

The longer lacquer crack, nearest the center seam, where it cracked beyond the cracked top, melted in with a couple of sessions of thinner/ retarder/ 10% lacquer. The top crack (visible) stopped at the tone bar. The lacquer crack beyond the tone bar is gone.

The shorter crack on the right was a different story.
[list=]Twenty-four hours after the solvent was applied, the crack was a little longer.

While applying solvent a second time, I watched the lacquer crack run, very slowly, towards the neck end and veer right.

Another couple of solvent applications (within a minute or two, along the new crack area) and the crack slowly ran out to the purfling.

With nowhere left to go, the crack line stopped.

Additional applications of lacquer/ solvent amalgamated the lacquer crack.

The lacquer cracks are now gone.[/list]

I tried this once before on a lacquer crack and it didn't work. The crack would run when solvent was applied. Even applying naptha to clean the area, caused the crack to run, you could watch it. It may be analogous to a crack in metal that will run until the end of the crack is "stabilized" with a drill hole. I also believe that the temperature differential of the solvent flashing off causes enough thermal stress to an already stressed surface to cause the crack to run.

This has been an experiment. I realize that scraping the finish off is a proper method of repair. If I end up having to do that in the end, then this work has caused no harm and will provide valuable information, even if it is only "scrape it off and start again, amalgamation will not work". With that said, the repair would have been faster if the cracked lacquer was just scrapped off.

TAKEWAYS:
[list=]Use naptha, not light, to check for plate cracks[/list]
[list=] For solvent repair, a drop ahead of the crack may prevent the crack from running- TBD[/list]

Don't leave a body standing on the side when workday is over

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Measure Twice,

Karl Borum

Use a small brush to wick some Butyl Cellosolve into the cracks.

Glu-Boost Fill 'n Finish is a better material for finish repair including cracks. I don't know of anybody who has tried it and not been a convert.

Just. Plain. Yuck!

After all the care, time, effort, and materials involved in building this instrument this far, I suggest OP back the finish off the damaged region,and prep and reapply finish where it was removed. A leveling and a respray of the entire top will return the project to its 'new. state, not a 'repaired' state. No damage, no workaround, no repair. Just lots of work.

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Peter Havriluk

That is where this guitar has ended up, scraping the entire top back to bare wood. The lacquer cracks continued to grow longer at the moment a "solvent" was applied. With a visor, I could see them slowly creep. When a new crack surfaced, in an area adjacent to the damaged area, I focused on it with Butyl Cellusolve (sp). The crack would not stop creeping and the experiment was terminated. I consider it a win, I will no longer attempt to repair cracked lacquer by amalgamation. Removing the lacquer back to bare wood would be the proper way to eliminate cracks.

On a side note, the cracked lacquer that appeared to be repaired still retained crack lines below the visible surface. I could see this while scraping off the finish.

In conclusion, my application of way too much lacquer was a contributor to this problem. I haven't yet learned when to stop applying lacquer, even following some very good advice from the OLF on the matter. THats for another thread I suppose. I'm switching to Target EM6000 WB lacquer.

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Measure Twice,

Karl Borum

Practice makes perfect. Make your coats as thin as possible to allow enough time to dry. When happy with your finish, let it dry for weeks until you no longer smell the thinner before buffing.

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

What Chris said and:

No more than three light coats in a 24 hour period.

Download StewMac'w Dread guitar kit instruction manual there is an excellent finishing schedule that is what taught me long ago it will help you greatly!!!

The StewMac schedule will detail how you sand much of what you sprayed yesterday off today before three more coats and so on and so forth.

No more than 9 - 12 total thin coats this is with Nitro lacquer but I would think translates to similarly viscous finishes.

And what Chris said no matter how much that 12" tall guy on your left shoulder tells you to not wait any longer before buffing out always wait 4 weeks for nitro lacquer to cure and more if you can.

Nitro and many finishes must be sprayed in 40 - 50% RH or you can get blushing from trapping moisture.

Please always observe safety precautions if using nitro, explosion proof fan, lights, no sparks, flames, or electronics in the area that could spark. Sorry Karl I know you know this I just have to say it in case someone else takes my comments and does not consider safety.

You'll get pickier as time goes on. When I had my separate paint shop with an enclosed Binks booth and all that, I would spray clad only in gym shorts, flip-flops, and a ballcap to keep the air clean. I heated my finish (real nitrocellulose or acrylic lacquer) in a water filled Crockpot to be able to spray more solids and fewer solvents to reduce overall time finishing (time is money). I wouldn't spray anytime the air was over 50% humidity, and I was using retarders, too. Most of the time I didn't have to sand out any orange peel, which is a time saver and it's less work.

My clients didn't like the wait, but they loved the results. In the 80's probably a third of my work was refins or new finishes. I was BUSY.

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

Chris, would mind to share your top couple of tips for getting that off the gun quality?

I’m still in the “it’s not how much you put on it’s how much you leave on” stage, but sanding takes time and money!


Sent from my iPhone using Tapatalk Pro

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My nitro finishes come out exceptional, except cracking.
I have Jeff Jewett's on-line course, the StewMac finishing book and the detailed LMO guide, among others.
I'm very methodical about the process, following all steps except I haven't been level sanding after every coat.
HALF the number of coats on the top

Mohawk Stringed Instrument Lacquer, 75/25 lacquer to reducer; different lots
Mohawk Reducer
Mohawk Retarder 5%
Rockler WonderFil WB grain filler
4 to 6 week hang time (two guitars cracked during or after wet sanding, before buffing)

The backs have never cracked.
First guitar the sides only have cracked at the shoulders, month 3
Second guitar cracked at week 8 after wet sanding, the night before buffing (scraped back to bare wood, reshot, no cracks)
Third and current guitar cracked falling off the bench onto hard concrete. Repaired shatter at impact point, top cracked away from the impact at week 12, during wet sanding the repair .

I've typed, deleted and retyped this a couple of times, its so much........ and I can't find a likely cause, except lacquer too thick. I dont know how to measure, but measuring a chip off the sound hole cover yields about .004" on the top (before level sanding...its the sound hole cover). THats where I'm at and it very discouraging, expensive and time consuming. I want to build guitars, not scrape cracked lacquer

_________________
Measure Twice,

Karl Borum

I forgot to add, I was VERY conscious of keeping it thin on this one.
Guitar 3 (this one that hit the floor) I sanded the sides and back down to smooth after every day's spray session (3 coats maximum).
HALF the number of coats on the top.
I wonder if it would have cracked if it would not have hit the concrete, I would hope I nailed it this time, I'll never know. Scrapping, scrapping scrapping......

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Measure Twice,

Karl Borum

Karl, the photo at the top of the discussion looks like an EXTREMELY thick finish which is definitely a cause of cracking.

Wonder if I should be reducing my lacquer more?

Using the "LVLP" gun from LMI / small gravity gun with 1.0mm filtered/ regulated 35PSI at the gun

https://www.lmii.com/filling-finishing- ... 88008.html

Sprays great, never orange peel, no runs no sags (well maybe once).

_________________
Measure Twice,

Karl Borum

Karl, do you know how thick your finishes actually are? This is a key data point that should help guide you. I found there were pitfalls for the unwary in many finish schedules, because “eight to ten coats” thinned 50:50 through a conventional gun with a small needle is much less lacquer than “eight to ten coats” thinned 75:25 through a HVLP gun with a fat needle. So you gotta know how much you’re putting on.

Easy way to check is to mask the site of the fretboard extension or bridge, spray the finish, cure for a few weeks, then pull tape. Using calipers to measure the tape and compare its thickness to an unsprayed piece of the same tape will give you an idea of how much lacquer you are actually putting on.

Too thick lacquer on the top is by far the most likely culprit as you have surmised.

Don’t be in a hurry, by which I mean, don’t be afraid to leave the finish thinner even though it increases the risk of blowing through when levelling and having to spray more. Pushing the limit in this direction will teach you how little you can get away with. If you blow through, spray more, wait more. It’s just part of the learning process.

Josh, hurry went out the door on my first one
Thanks for the guidance.

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Measure Twice,

Karl Borum