Hello Boltnut,
Old water-slide decals can be tricky sometimes, that’s for sure!
You might want to check with your local hobby shop for some stuff called “Micro Sol” and “Micro Set” made by Microscale Industries, Inc. These are mild solvents that slightly soften decals so they conform to the surface better. If your decals are loose, you might want to test this on a small area first. If it works, carefuly brush it over the decals in one stroke . . .do not touch the decals until they are dry. You might even consider spraying it on using an airbrush with low air pressure. When applying new decals, “Micro Sol” and “Micro Set” usually works great. The decals will go over irregularities much nicer and won’t have the “silvering” effect caused by air trapped underneath them. Hope this helps! Please let us know what happens, okay?