OK, some thoughts (and things I've learned!) about decal sheets:
For starters, decals (incidentally, the word "Decal" is a contraction of the original name "Decalomainia" which dates from at last the 1920's or so), are multilayered in their manufacturer:
The "decal paper" on which decals are printed is basically "blotting paper", which is highly water-absorbent. Waer penetrates this paper from behind, NOT through the decal itself. The "glue" (adhesive) traditionally is just plain gelatin (as in Jell-O, albeit with no flavoring, sugar, or food coloring).
Good quality decals are printed in lacquers, with a clear base (either printed in the shapes of the artwork, or sprayed all the way across the entire sheet. Commecially made decals are printed, most often in lacquers, the succeeding colors printed on as layers, in solid layers, as opposed to those old cheap model kit decals we used to see years ago, which more often than not were printed by lithography (which gives an uneven coloring, looking like either "cross-hatched" or "dot matrix".
It can, and has happened, particularly in mass-production printing of decal sheets, that the first step in printing the decals somehow the clear lacquer basic coating got missed--but that is very easy to spot--if you look closely at the decal sheet, and see the gelatin adhesive exposed, with NO clear film around the edges of the various artwork, it is time to STOP! The lacquer printing itself is nowhere nearly strong enough to withstand being wetted, and then "slid off" without merely crumbling into tiny bits. So what to do?
Whenever I encounter a decal sheet that is either very old, or appears to not have been printed over a clear film applied to the decal paper, I have simply airbrushed a coat of clear non-penetrating lacquer over the entire sheet. Clear "non penetrating lacquer" is as readily available as can be: Model Master gloss clear lacquer, as well as Tamiya TS-13 Gloss Clear!. Are the decals from an old kit? If so, they can be prone to splitting apart due to their age, and quite possibly some curling of the decal sheet (blotting paper absorbs moisture, and it's at all particular where that moisture comes from, be it the dish of water you dip decals into, or from the humidity in the ambient (surrounding air). So, again, clear gloss non-penetrating lacquer (as described above) comes to the rescue, and it can (and in my decades of experience) does preserve such older decals quite well.
From late 1966 through the winter of 1983 I built (along with several local model car friends) a large number of Indianapolis cars--most of which required decal graphics that did not exist in any form whatsoever. For these, I used both scraps of decal sheets (the clear parts left over after cutting out the kit decals) and even raw decal paper having the gelatin glue coating but not even clear film--simply spraying (back then AMT Clear Lacquer--which was about the same thing as Tamiya or MM lacquers) to get that clear film laid down--and then literally hand-painting graphics such as car numbers, even driver, car owner, and sponsorship graphics. On more than one occasion I even airbrushed colors used in the basic paint scheme onto clear decal film, then cut out my own shapes in order to get some of the more complicated paint schemes into areas of the body work, where masking and spraying was just not possible. For hand-painting smaller decals, such as car numbers, even down to car owner logo's (such as JC Agajanian's legendary "pig riding a race car", drawing inspiration from an articles in the likes of Car Model, and Rod & Custom Models on this subject. I've even changed the colors of such things as race car numbers, and cleaned up the cheap lithography on old 1960's era AMT and MPC decal graphics.
None of what I'm saying in this long-winded missal is at all highly technical--it's easy, but with the caveat that there are "learning curves" along the way, particularly with hand-painting--but it is all doable, I've both done it, and have seen others do it, all along the way.