I have always read that water-slide decals were to be dipped in lukewarm water in order to loosen them from their backing. But it doesn't matter - lukewarm water gets cold really fast when it is sitting out around here.
I'm a bit reluctant reluctant to handle soft water-slide decals with a hard metal tool, even a very smooth one that is designed especially for that purpose (decal tweezers). Using a small soft bristle brush just seems to be a bit safer to me, but I suppose it's a personal preference. The soft bristles handle the decal just fine. I don't know what you would want a stiff bristle brush for, though.
I couldn't agree more.
Nearly all of the decals that I use are reproductions from Excelsior that have been produced with an Alps printer. The image is not quite as hard and durable as it would be if it had been screen printed. They are excellent quality decals, though, and I am delighted to have them. The coating I described is recommended by Excelsior. I have not experienced any damage to any decals that have been overcoated.
Screen printing ink is not technically "paint," although some types can closely resemble it.
As I recall from my model-building days, decals applied to painted styrene models are generally quite durable without the need for overcoating. But then, styrene doesn't absorb humidity, and styrene models do not undergo the same stresses that a model rocket does. Nevertheless, I would expect that decals that have been applied to hard, stiff and smooth surfaces in a model rocket will hold up very well. Unfortunately, many of the surfaces that receive decals on a model rocket are not quite as dimensionally stable as molded styrene. Because of this, after some time and use water-slide decals on model rockets can eventually crack or flake off in parts; it's just due to the nature of the material that we use, which become the substrates for them. Coating them can help to delay this process, especially for decals that have been Alps-printed. As has been discussed, there are a few different types of coatings that can be used, but I don't think that leaving decals (especially Alps-printed) uncoated on a model rocket that is made primarily out of paper and wood would be such a good idea, unless I was keeping the rocket in a display case and never flying it.
I can't say whether the same would be true for screen-printed decals, though, because I have hardly ever used any. Back in another lifetime I was a printer in a screen printing shop that produced, among other things, assorted shapes and sizes of decals, but I have rarely had the opportunity to use them on model rockets.
Vinyl decals (real ones, not stickers) are reputedly very durable, but it is not practical (or even perhaps possible) to make vinyl decals for rocket airframes that are smaller than about 3" to 4" in diameter.
MarkII