Checking a finish on purpose

[Smyers]

I never thought I would purposely ask how to do this, but here we go.

I recently purchased an old flatback mando circa 1915. #It required a bit of restoration work. #A sloppy coat of shellac has been slapped on it at some point of it life. #The mechanical work should be finished soon and I need to think about "fixing" the old finish.

What my current intention is to wet sand out all the lousy runs in the bad old "overfinsh" job and thin up that finish a bit. #Then, I need to refinish part of it properly, perhaps by varnish/french polish. #The headstock is the only part that look authentic and completely original in finish. #The headstock has a very fine checking pattern in it one would expect for an instrument of this vintage that has been played as much as this one has.

My question is how one would make the new finish check on purpose to similate age. #If there is a way, can one control how wide or fine the checking pattern is? #I intend to keep this thing in the family, but I want it to look period. #With all the "distressing" that is done these days on new mandos, I am sure someone here is privvy to how to prematurely check a varnish/french polish finish.

[Paul Hostetter]

If I'm not mistaken, Dan Erlewine has an instructional video on just this sort of thing, available at Stew-Mac.

[8ch(pl)]

I think you may be able to use denatured alcohol to remove the bad shellac.

[Rick Turner]

A fast freeze will do it. In the old days, we'd use Freon...

[Paul Hostetter]

Maybe freon is a little outré these days, but there's probably something like it that won't further trash the ozone layer. I would point out that spot freezing is intended for nitro lacquer and may not work on other finishes. Never having tried it, I can’t really say. I’ve seen the results and they look pretty cool.

I think your stated intention to wet-sand the lumps is much preferable to trying to deal with them chemically. Denatured alcohol is kinda toxic to begin with, and shellac is a good underlayer for whatever else you’re thinking of putting over it. You may in fact not need any other finish if you can take it down carefully.

[hotclub]

I've heard of using dry ice.

[Smyers]

Yes, shellac via french polish will be how I will most likely finish it, as this is how it was originally finished to be sure.

Interesting idea on the freeze. #I do have dry nitrogen tanks available to me. #I can put the instrument into a leaky box and diffuse nitrogen into the box so it doesn't hit the instrument directly, while monitoring the temperature with a thermocouple to avoid too cold of a temperature. #Sounds promising, not too difficult and most of all, controllable. #I could do something similar with dry ice too. #Or, just rig up a tube and gently spray the escaping nitrogen onto the surface of the instrument, probably much like you did with the freon. #It's going to require some testing on test pieces no matter what I find works. #You've got me thinking.

I had thought about trying to chemically "pickle" the finish too, but this has some serious risks if it is overdone and involves nasty solvents. #Control of such a process would be much more difficult and would probably involve careful spray with an airbrush to not overdo it. #The advantage of this method is you could just put it exactly where you want it with little to no overspray.

I'll look for the video. There may be something really simple to do in it.

[Desert Rose]

Fender Custom shop uses CO2 cartridges on their Relic series

Applied at a key point after spraying the finish it check wonderfully and simulates real checking perfectly

Scott

[ErikAitch]

Just a thought from the furniture repairman in my brain: if it really is a sloppy shellac job, I'd reamalgamate it with denatured, removing as much on as needed in the process. Who knows? maybe there's a nice checked lacquer finish underneath the glop.

[Rick Turner]

I'd bet a grand that there is not an original 1915 lacquer finish under whatever is on that thing. Up that to two grand, and kindly pay up!

That's shellac or varnish or both. Think about this...sprayed lacquer wasn't happening until the mid to late 1920s, and that was when DuPont developed the process for Henry Ford. There might have been some nitro lacquer being used in Germany slightly earlier, but not by a hell of a lot. And DeVilbiss was the one who developed the modern spray gun... http://www.autorefinishdevilbiss.com/binkshistory.php

[Darryl Wolfe]

You are a wealth of info Rick..thanks

[amowry]

You can get cans of compressed air (like airbrush propellent) that might do the trick. There was a discussion about this recently on mimf.com, but I'm not sure if it's still there.

[testore]

Older shellac is more difficult to crack, but it can be done. You can use hide glue and a hair drier. You can also make a egg albumen cracking agent too.Just whip up an egg let the albumen settle at the bottom and pour it out.It has a varnish consistancy and it lays where you want it to. I use a hair drier to get a more intense, deeper crack.It sometimes takes two applications, especially older shellac. If you level the older shellac and put a new thin layer on top it will slighlty soften the older layers and help crack it with either of the above two methods.There used to be a crcking varnish available in hobby stores but I haven't been able to find it for years.