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BTW I am getting all kinds of security warnings visiting this site.

I've never had this problem before though I did change my schedule this time. French Polishing is a very physical finish to apply as anyone of you who does it knows. Typically I would pore fill then brush on a few coats before polishing to help speed up the process. The only thing I did different this time was spray on the first coats, and use Zinsser.

I have already done one guitar this way and it didn't have such problems.

Process:

1) pore fill with Aqua Coat sand back and wait a day
2) spray 4-5 coats Zinsser Seal Coat 1.5 hours between coats straight out of the can
3) sand back flat
4) spray two coats super blonde
5) sand back flat
6) French Polish with super blonde to build back the shine
7) 3-4 weeks later polish out with McGuire's compound

It looked great. Then the holidays hit and I did some traveling only to come home to this. My shop is 24/7 controlled perfectly too.

I sanded back through the cracks and French Polished again but I've lost confidence.

Thoughts?

I’ve never seen that in shellac. My guess is that you built the thickness too fast with the spray can. When I’m doing one coat, I rub a 2 lb cut. When I started french polishing, I would rub a 2 lb cut twice a day. Any faster and it wouldn’t build because the 2 lb cut didn’t dry fast enough. Now, I use a 1 lb cut and it’s usually dry enough to start recoating in 20 or 30 minutes. So, I can get a much quicker build with the more dilute shellac. If you must spray, you might want to wait longer between coats. My guess is that the can is pretty shellac dense giving you effectively more than a 2 lb cut. I’ve used Zinsser’s spray shellac but never as part of a finish, only to get shellac somewhere quickly where it might be removed later. Zinsser’s uses more than ethanol as solvents with the other solvents varying a bit by product. I don’t know how that would affect drying times.

My best guess, which aligns with Bob's thoughts, is that there is something in the Zinsser spray shellac that wasn't done gassing off before you re-coated via the French polish method, so your new coats were drier and harder than what was underneath. But who can know for sure? Not having a complete knowledge of what they put in the spray can, I would probably only use it for stuff I want to spray and leave alone. For re-coating, I would want to go thinner and be in control of the chemistry. Sorry this happened!

JF is the crazing under a top layer that is still smooth? It's difficult to tell from the pics so can you feel the crazing with a finger nail in a safe place to test this?

If not and the top layer is not crazed the sprayed layers are suspect. When you say you sprayed Zinnser was it rattle can or did you cut it to 1lb and spray it. I never used Zinser at full can strength and always cut it to 1lb.

I have seen FP shellac craze and it's not all that uncommon either I've seen it several times that I can recall. It was always at least partially related to building too much too fast.

Wanted to add for others without experience who read this thread and want to learn we are speaking of Zinnser's dewaxed right?

I have used (and thrown out) a lot of pre-mix shellac, but not on an instrument.

Have you checked the date code on your can?

Zinsser now specifies their Zinsser® Bulls Eye® Shellac shelf life as 3 years (wow I think that is a stretch...). I guess you did a drying/ hardness test on "glass"?
https://www.rustoleum.com/-/media/89B53 ... 810610.pdf

I was looking for their date codes, nowhere to be found on their site.....

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Measure Twice,

Karl Borum

I would mix myown, Zinnser is not pure sellac.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I think you might be right about the thickness of the coat in general but I should have mentioned I'm using an HVLP set up with a spray gun. I'm not sure what cut of shellac Zinsser is out of the can. I don't, and never have, measure it myself when I make it. I simply put flakes, or crushed up buttons, in a jar then cover it with alcohol so that it's about twice as much alcohol as flakes.

Yeah I can feel it with a fingernail so it's the top layer. As a former geologist I would equate it to what looks like mud cracks. The Zinsser is the dewaxed stuff the date indicated that it was about 6 months old. So I was wondering if it was the super blone on top of the zinsser. But maybe it was build up too fast too.

I'm hoping now that it's has cracked and I sand back to the bottom of those cracks it won't happen again? Wishful thinking maybe.?

I know some luthiers use it. I was reading some on Delacamp that exclusively use it. It probably has some weird additives in it but it does at least claim to be dewaxed shellac. I think acetone is definitely in it.

I had that issue on two guitars and a mandolin when spraying with Royal Lac. After some back and forth with Vijay it turned out my issue was applying the shellac to thick and not allowing sufficient time between coats. I changed my schedule and have not had the problem again. I've never seen crazing when I've done seal coats with Zinser so I don't know if that has anything to do with what you're seeing or not.

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Steve Smith
"Music is what feelings sound like"

Ok yeah that's probably it. I was doing more than seal coats, I was building it with the Zinsser up to 5 in total 1.5 hours wait time between.

It scuff sanded nice and dry though. I guess it has other volatiles in it.

Just FYI, In my experience Seal Coat is not nearly as durable or as hard as flake shellac. I don't know what's in it but I won't go near the stuff.

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Stay with the happy people.
--Reynolds Large