Don usually recommends scanning your decals (especially the old ones) before touching them. This way, if something goes wrong, you can re-print them. I have already made several custom decal sheets using CorelDRAW for drawing and custom printing companies for the print itself.
While it probably isn't heplful in your case, mabe re-drawing your decals would be less work than experimenting with all that chemistry?
The biggest set I made was for my Whale:
Here I used amix of custom made and kit decals. There were moments, where I regretted not re-drawing all the decals, for example when the wing walkway (kit decal) warped in an ugly way in process of putting it on. The custom-printed decals went on like a charm.
Hope it helps, have a nice day