I strip clean and degrease my cc's guns and coat them with spray polyurethane. I do that every couple years. The cc on my guns, used continually year round show no degradation of the colors whatsoever since coming to my care.
So use and do whatever you like but if you like the colors to stay there are FAR better coatings than tung oil, laq (watta a joke!), or varnish.
I have been told of a bake-on enamel that Brownells sells but have not had the opportunity to check that out. Bake-on has some great potnetial for sure.
The baking lacquer from Brownell's is tougher than hell, tougher than catalyzed poly, in my opinion. You need to own, and be qualified to use, spray equipment, to apply it. I'm not sure what would cut it, if you decided to try to take it off. Never tried, but, standard dip type carb solvent would be worth a shot, and does not hurt case colors. You can clearly tell when a gun has been coated with this stuff, not all guys like that look, but, I prefer case colors where the factory put them, and most factories put something on them to try to keep them around.
I've used the spray bomb Krylon sold in art stores, which is just a thin lacquer, dries fast, and protects well enough, and is easily removed with a dip in a pan of lacquer thinner.
Most of the spray bomb polys sold in the stores turn an unfortunate shade of yellow, when exposed to sunlight. I have a few older cans of Deft and Guardsman poly, that are clearly marked "For indoor use only", and I'm pretty sure they were water resistant enough, but, suffered phase shift issues when exposed to direct sunlight. The baking lacquer, lacquer, and automotive type polys that are sprayed out of a gun, do not. Neither does the Krylon spray can stuff, as it is aimed at artists who use it in applications where that would be a huge no-no. It is thin, and I would say just effective enough.
I have a quart can of Duco "Crystal Clear" lacquer from the bad old days of automotive refinishing, that I have used on case colors a few times. It is tougher than tung oil, clear, forgiving, and impossible to buy anymore.
I'd say it is as durable as it needs to be for this application, and wish it was easily obtainable.
More than once, I've had trouble with fisheyes, and wrinkled coating when spraying guns, as people seem to think silicone is a proper lubricant or rust preventer. I like to clean twice, once a dip in regular spray gun cleaner grade lacquer thinner, and a second time in automotive wax and grease remover, neither of which will harm case colors, unless you start rubbing on them.
I sprayed baking lacquer on this Darne R10 21 years ago, and have used it relentlessly, every season since, in all kinds of weather. Still about the same as the day I did it:
On the other hand, I wish I could come up with something that would prevent this:
This is a mid to late 1960s vintage Darne V19 that I was patterning on Saturday, and when I got home I discovered this chunk of horn missing from it's tip. Hindsight being 20/20, I maybe should have waited for warmer temps, and in the future I will do so with horn tipped Darne guns.
The gun has a long LOP, and won't be used by me during cold temps anyway.
I imagine this will be repaired with tinted Acra Glass, and look fine when it is over. But, that isn't what I said when I saw it as I slipped the gun out of it's case.
Best,
Ted
