Tom 'n Ray - gentlemen, I've long admired and have benefited from your Flyer wisdom -- and in this case, I won't say you are wrong, but this is one of those safety conundrums -- "product X, when used safely, is safe". I looked up the MSDS for gumout (the 'ordinary' kind) and its mostly (60-100%) light distillate, 1 to 5% middle distillate, 1 to 5% naphtha and < 1% naphthalene (they don't give you the exact fractions, claiming trade secret). The latter two are certainly flammable, with the flash point for naphtha at -7 F (that's a minus sign) and for naphthalene, its 176 F -- the distillates are likely harder to ignite, but in the presence of naphtha, I'm sure that ignition isn't a problem.
So letting the mixture sit probably helps reduce the amount of naphtha in the mixture, still...
I wonder if something like gumout keeps enough of the deposits in a smokebox dissolved long enough to be volatilized or whether, at the end of the day, they simply redeposit after the gumout is gone (unlike the case in an automobile, where the gasoline containing the stuff the gumout has dissolved goes into the combustion chamber).
I make no claim about being enough of a chemist to be very certain about what the margin of safety is for this but I did spend a career as a scientist at a National Lab and these were the kinds of 'McGiver' approaches that would sometimes give lab safety folks apoplexy...
- Rich