I feel your pain, as I’ve suffered the same wrinkled water-slide (WS) issues too many times in the past. Assuming the surface to which the decal has been applied was squeaky clean (no wax, no debris, no contaminants and prepped with 2000 grit paper), the WS was allowed to dry for at least a full day, preferably 2, you should be in a good starting place.
I note that many scale-model builders are adamant about using a “setting solution” during the WS application and claim this provides superior application/adhesion of the WS. One such setting product I am aware of is Walther’s Solvaset, which in my experience is a sufficiently strong solvent that it is clearly capable of wrinkling/distorting a thin WS decal. I would steer clear of this product for thin WS decals.
https://www.walthers.com/solvaset-decal ... 1ml-bottleAnother setting solution is Microscale’s MI-1, which to the best of my knowledge is a dilute solution of acetic acid (white vinegar) and in my experience, when used in minimal quantities does not distort WS decals.
http://www.microscale.com/Merchant2/mer ... ct_Count=1The setting solution discussion above may have merit or it may be all smoke and mirrors, I clearly do not know which, but what I do know is that all my WS failures have occurred when applying clear, and more specifically, the clear upset the WS by swelling and distorting it to various extents. Perhaps this was because the WS was not adhered perfectly, but my intuition is that regardless of how perfectly the WS is layed down, you simply cannot apply a solvent-born clear that swells/attacks the decal and expect good results.
I had one painter tell me that his clear was solventless, (which of course it wasn’t) and it ended up damaged the decal like all the other clears have (small bubbles/pimples much like Psychwarlord described above).
I am not suggesting that I have an absolute solution for clearing WS decals, but I can offer the following thoughts, all of which focus on minimizing swelling of the WS.
Minimize solvent contact time by using the fastest solvent available
Minimize clear concentration to avoid thick, wet, slow drying layers of clear that can swell the WS
Use the fastest curing clear you can get to minimize the time between spraying and a cured coating (the cured coating can't hurt you but the liquid coating certainly can)
Use an airbrush for the 1st and 2nd clear coats to absolutely minimize the amount of material deposited (just a dusting each time) and allow it to cure well between coats
Once you have a well cured, non-swellable layer of clear sealing the surface of the WS, you can pour on the clear to whatever extent you want since it can no longer swell the protected WS.