Shellac finish question

[JeffHGW]

Hi everyone

I'm doing a shellac finish for the first time and having an issue with what appears to be crazing. My application was I wiped on a bunch of thin coats of shellac over time until there was enough buildup to level sand and buff. It looked good for about a week, and then I noticed some micro cracks starting to develop in a few places around the top only. The back and sides seem to be ok.

Does anyone have an idea of the cause, and possible solution for this issue? Ideally I'd like to repair rather than re do the finish. And next time avoid it entirely.

I've attached a photo, not of my guitar but of one that has a similar looking issue but more sever than it appears on mine. You can see it around the reflected light. On mine it looks like the beginning stages of this sort of thing.

Thanks

Jeff

[kiwigeo]

Jeff, whats the shellac mix you're using? Is it hard shellac or conventional shellac? Whats the cut? What sort of oil are you using?

How long are you waiting between bodying sessions

[JeffHGW]

It's a 2 lb cut, conventional dewaxed platina shellac flakes from shellac shack. No other additives. I didn't use any oil. I wasn't doing a traditional French polish of smoothing the finish using the pad, I was just wiping on successive coats to get a build up then sanding/buffing. I started out doing 4 coats in a row, just going over the entire guitar once per coat applying it wettish, 4 times in a row. In between lots of 4 coats, I was level sanding a little bit and then waiting 2-3 days for the next lot of 4 coats. After a 3 sessions of that (12 passes over the guitar) I changed to waiting 2 hours in between each coat. Still 4 in a day. I did 24 coats in total. But with the level sanding in between every 4 coats, it ended up pretty thin. about .002" measuring in the bridge area.

[kiwigeo]

I started out doing 4 coats in a row, just going over the entire guitar once per coat applying it wettish, 4 times in a row. After a 3 sessions of that (12 passes over the guitar) I changed to waiting 2 hours in between each coat. Still 4 in a day. I did 24 coats in total.

It's possible your applying to much shellac in a session (particularly during those early sessions) and not waiting long enough between sessions. I'm no french polishing guru but I never have the pad wet and I don't do more than two bodying sessions in a day.

Crazing is usually an issue where layers are not properly amalgamating with earlier layers. With a damp pad and a lubricant you can apply significant pressure during a bodying session which aids in amalgamating the old and new layers of shellac.

[tapslider]

You can get crazing with most finishes and the usual culprit is surface tension so in agreeance with the last post you maybe applying to thick a coat try reducing to a one pound cut. I don't use oil in the early stages as this can cause soft polish as shellac loves oil and if not careful it get trapped in the finish. Time stands still when your french polishing have fun
Regards Tapslider

[tapslider]

I'll add to my last post after re reading your post applying 4 coats in a row of a 2lb cut is way too much too soon

[JeffHGW]

Ok, thanks for the advice! I'll be sure to apply more slowly the next time. Do you guys think it would be possible to re amalgamate the cracks on the existing finish?

[ozziebluesman]

Hi Jeff,
I have been doing shellac finishes on my stringed instruments for years. My finish is not a French Polish but layers of shellac to achieve a good coverage of shellac to be able to cut back with sandpaper between sessions. I use dewaxed blonde in a 2lb cut. when applying I have two syringes one with the 2lb cut and the other with pure ethanol. Usually I do four times around the guitar in a session using a rubber and a squirt of shellac and ethanol to the rubber.

The environment you are working in makes big difference to your success and needs to be 60% or less up here in Townsville. Sometimes the session of four layers takes a few days to dry or even a week sometimes. Rub back with 600 grit and go again. It usually takes me about 8 sessions to get the amount of product on that will look good after sanding back. I then store the guitar in the dry box at 45% humidity for five five weeks for the finish to cross-link, dry. Works for me. Martin is spot on, 2 lb cut on it's own is too much and that is probably where your crazing in developing.
Cheers
Alan

[kiwigeo]

Do you guys think it would be possible to re amalgamate the cracks on the existing finish?

The crazed layers haven't amalgamated with underlying layers so really you need to remove the crazed layers down to competent layers. I'd be cutting back until you get a nice uncrazed surface and go from there. As Ive noted I think you need to be aiming at not applying too much shellac per session. I aim for a damp pad....I test by applying pad to some paper. Pad should leave dots of shellac on the paper rather than patches. Start bodying session with light pressure on the pad and increased pressure towards end of the session. As noted I do a couple of bodying sessions in a day.....there's always at least an overnight wait between every two sessions. You can probably decrease that wait period but I prefer a slow and sure approach.

[ozziebluesman]

Yep, I agree with Martin.
Cheers
Al

[JeffHGW]

Alan, so when you mix 2lb cut and ethanol on the rubber, they combine and have a similar effect to using 1 lb cut? Is there an advantage to applying each to the rubber separately? Do you vary the ratio in certain situations? Thanks.

[ozziebluesman]

Hi Jeff,
Yes that is exactly what I do. The advantage is if the rubber becomes sticky just add a little, 2 drops of ethanol and keep rubbing a little longer till it stops sticking and re-load the rubber for the next patch usually 100mm x 100mm area. The big necessity is to let the instrument finish dry well before starting a new session. Another thing is I don't use is any oil at all. Just the mixture between the ethanol and the shellac mix. The best thing about living here is 60% humidity is usually with 30 degrees and the shellac dries nicely.
Cheers
Alan

[kiwigeo]

LOL..oil versus no oil. I'm in the oil camp.

I've tried all sorts of oil but these days I generally use walnut oil. I use as little oil as is required to allow me to apply some serious pressure and not have the pad get sticky.

Anyway.... stick with Allan's sagely advice since you're already in the no oil camp.

[ozziebluesman]

But my finish is not a French Polish as such. My old joints and bones won't allow me to push the rubber hard while applying a shellac.
So I call my method a shellac finish not a French Polish as such where you definitely need some sort of lubricant and many. many hours of hand rubbing.
Cheers

Alan

[JeffHGW]

Here is the crazing on my guitar. It's really only noticeable in certain lights and was hard to capture in a photo. My plan is to sand it down until it looks like the cracks are gone, then re build up the finish more slowly following Alan's method. I wonder though if there's a risk that the lower levels of shellac would possibly appear ok, but would also be prone to crazing. Maybe I should go back to bare wood?

[Jim watts]

Have you considered going over it with just alcohol on a pad and some pressure? It would be like spiriting off in a French polish routine. If it’s really only .002 thick it might do it for you.
It’s hard for me to imagine 24 coats of 2 pound cut being only .002 thick, but I’ve been wrong more than once, it’s hard to admit though.
Nothing to loose really since you’re considering sanding back anyway.

[JeffHGW]

You make a good point Jim, I'll try that first. There was a bit of sanding in between coats which made things thinner.