Posted by John Swensen on July 10, 2002 at 11:41:04:
In Reply to: Re: Cooking oil posted by js on July 09, 2002 at 21:43:54:
When cooking oil gets gummy, then hard, it is, actually, turning into varnish through oxidation (not very good varnish, but varnish, just the same). I have been fortunate enough not to have encountered cooking-oil varnish in a brass instrument, but I occasionally end up cleaning it off cookie sheets, waffle-irons, etc.
ALL OF THESE TECHNIQUES REQUIRE FULL EYE PROTECTION (WRAP-AROUND GOGGLES, AT A MINIMUM)!
The good news is that these kinds of varnishes are not too resistant to solvents, but the bad news is that most solvents that will attack a varnish will, eventually, attack lacquer. A friend of mine swears by simple lacquer thinner to strip varnished woodwork, and that might not attack modern, epoxy-based lacquers (some soaking and scrubbing is probably necessary). I have had good luck using paint strippers (3-M's Safest stripper is almost safe enough to drink, and attacks varnish in a couple of hours); if you can get the gummy valves apart, you could paint this stripper inside the valve casing and on the valve, let it sit overnight, then scrub out and wash the next day. Any stripper that found its way onto any lacquer would remove the lacquer in that time.
For hardened varnish that has glued a valve in place, perhaps laquer thinner could be dripped into the top and/or bottom of the valve, into the tubes leading to the valve, etc., until the valve freed up enough to remove it without damage. I would not use lacquer thinner anywhere but outside of all structures, however, as it is very nasty and stinky stuff (the last time I used lacquer thinner for less than 5 minutes in my garage (with the door open!) I saw stars for an hour afterwards).
Going up a level of seriousness, there are thin, sprayable, paint strippers that might wick their ways into the valve if applied carefully (eg. plug up the bottom hole in the piston valve with something, then fill the bottom of the casing with stripper (casing upside down), and wait for things to loosen up). Unless you have magical powers, large sections of lacquer are almost guaranteed to be destroyed during this process. Once the valve is out, the casing and valve probably need follow-up stripping of the residue.
I have used caustic solutions to remove old paint (both boiling TSP (trisodium phosphate) and lye (sodium hydroxide)), but the bits of brass that were present ended up pitted afterwards (I don't know what they looked like before the stripping began). I would test a piece of brass tubing from a hobby store with the caustic stripper (TSP, lye, oven cleaner, etc.) before trying it on a horn. Maybe epoxy lacquer would hold up to caustic strippers, but they might be damaged or dulled in the process. Boiling temperatures would, almost certainly, convince any lacquer to part company with the brass underneath.