I learned an important lesson about using DIY decals. Taking a close look at some red cross decals that I had printed, using Kodiak-brand clear decal material, on the model of the hospital ship U.S.S. Repose that I am building, I noticed that one of them had rough edges. It was clear that the soft paint brush I had used to position the decals had damaged them; the problem was that I had trimmed the red crosses right up to the red, which left the extremely fragile decal film vulnerable to damage.
I removed the decal (not easy — those decals are tough once the adhesive has dried) and cut out another one, leaving perhaps a .5mm border of clear decal film. It went on without a problem, and the clear border — undamaged! — is virtually invisible.
I normally spray my decals with three coats of clear Tamiya spray, waiting a few hours for each coat to dry, and then apply them a few days later. The red cross decals had a good two months to dry. I’m pretty sure that the damage resulted from cutting them too close.