Cracks in Finish

[Selah]

Hello Forum Friends. It has been a while since I have posted. I built by first Martin kit back in 2020 and am doing two more for my boys. I am now doing the finishing and did everything the same as my first guitar following the Stewmac Guitar Finishing Step-by-Step book. My first coat was a shellac (Zinsser Bulls Eye). My finish was ColorTone Nitrocellulose lacquer from Stewmac. Finish was applied per the Stewmac guitar book like this:

Day 1: spray lacquer three time, two hours apart
Day 2: Sand lightly with 320
Day 3: spray lacquer three time, two hours apart
Day 4: Sand lightly with 320
Day 5: spray lacquer three time, two hours apart

I was unable to spray exactly on day 5 and had to spray day 8, but I still sanded between coats.

I bought the nitro about 8 months before I sprayed, so it had been around for a while.

Somewhere between the day 3 and day 5 spraying application, I noted what I thought were scratches appearing in the finish. I sanded them down and resprayed. After the last application, everything looked great ... now the problem. I let the guitars rest for about three weeks and just went out to wet sand with 800+ grits and buff the finish. When I did, I noted what looked like scratches in several places on the guitar. Turned out they were not scratches, but cracks in the finish! I was horrified. After looking at the bodies and necks very closely, I found the cracks in many places! I literally felt sick to my stomach.

Guitar Finish.jpg

Any ideas what caused it? Any suggestions on how to fix it? I am honestly thinking I will need to do a complete removal and reapplication of the spraying.

Thanks for your help.

Jim

[Diane Kauffmds]

Jim, were they subjected to very warm conditions and then cold?..Expansion and contraction can cause checking in the finish.

I've read that sanding too smooth, by using a high grit l, can cause cracking. I'm more experienced in shellac, although I spray lacquer on occasion. John Hall most likely has the answer to this.

[tippie53]

did you use dewaxed blond shellac most shellac sealer is mixed with shellac flake and ever clear
Temp or if you sanded too fine.
You can mix retarder and thinner and scrub the area and respray
don't use stewman go to home depot and get minwax

[TEETERFAN]

To my eye, it looks like the top layer of lacquer dried much faster the the first layer(s). Kinda has the “pie crust” look. Your timing seems correct (spraying on the 8th day instead of the 5th day shouldn’t matter). It looks like a lot of material on the instrument. My guess would be the first layers ended up being thicker than they should have been and weren’t as dry as needed for the time schedule. New layers of lacquer will partially dissolve the first layers because the solvents both dissipate into the atmosphere and migrate into the previous layers. The shellac does a good job preventing the lacquer (and its solvents) from soaking into the wood, but that means the overall drying of the lacquer coats is slower, since the solvents have to evaporate into the air to dry. That said, your last layers may have been too heavy and the additional solvent re-softened everything. The top layer skinned over and cracked as the inner layers dried and contracted.
And perhaps your last coat dried faster due to conditions such as higher temperatures or lower humidity. My advice would be to sand out the cracks and either lighten your application of lacquer and/or increase the drying times between coats. I am always careful about building too thick of a finish too fast.

[Diane Kauffmds]

I know you probably think this is a disaster, but it's not. You made a very good choice to use shellac and/or nitro. Both of these finishes allow correction down the road to fix things like this, without stripping the finish.

These finishes come with a bit of a learning curve. Don't be discouraged. This is fixable.

As Teeterfan said, and it crossed my mind, that maybe your nitro coats were a bit heavy. Also, as John Hall pointed out, make sure you use dewaxedshellac. If you use a prepared commercial shellac, like Zinsser, make absolutely sure it's dewaxed (it'll be on the can). When you use 2 different finishes, like shellac + nitro, you have to make absolutely sure that the shellac is really dry, before going onto the nitro. Shellac can be dry to the touch, but not really set up enough. I'm not sure about nitro, but shellac shrinks as it sets up. If they do this at different speeds, it'll cause cracking.

[Selah]

Thanks for the tips so far. Here is the shellac I used:

D2E1D137-CFB4-4DA4-AB94-8487A2F67174.jpeg

2E5432C5-28F7-4064-BED9-26FA9299EB5E.jpeg

It doesn’t say it has wax, or doesn’t.

[Selah]

I am wondering if the cracking will stop. Can I sand down the cracked areas and then reapply nitro? Do I need to reapply shellac then nitro?

The temperature when I did this was around 55 in my shop. But it never got too hot either. I have a dehumidifier that keeps my shop right near 61% humidity as I live in a wetter climate.

[Kevin Sjostrand]

Hey there.
So first off the shellac you used has wax in it. What you need to be using from Zinsser is labeled "Seal Coat"
That product has no wax, just shellac and alcohol. Maybe another thinner but I don't think so. Many folks have made this mistake soyou are not alone. This might be your issue and if so, you may have to strip and start over.
If you can't find Seal Coat then you can get "dewaxed flakes" and dissolve in either pure distilled alcohol or what I use is denatured alcohol. You can also cut the Seal Coat with alcohol. I think out of the can it's a 3 pound cut.
Hope this helps you. It is a bummer.

[phavriluk]

The fastest way to come to a good end is to avoid attempts at interim fixes. I think Kevin's got it right, strip back to bare wood and start over with the shellac Kevin mentioned. Yes, stripping is time-consuming, but OP will also be able to tend to those little areas that got overlooked the first time (every one of us here has pushed on whe we should have paused and tended to something).

[Diane Kauffmds]

Zinsser universal sanding sealer, called SealCoat is 100% wax free shellac. Unfortunately, their shellac traditional finish has wax. The finish really needs to come off to be redone. You're asking for trouble down the road if you don't.

Be very aware that sanding the finish will create little round "pads" of hard finish on sandpaper. These tiny round things will scratch. your guitar. Even if you strip the finish, there's always a little left on the wood. Frequently stop and check your sandpaper. You can clean these pads off by scraping them off.

Just make sure your sandpaper stays clean.