Help! Torn decals

Decal set (not solvent) will loosen the damaged ones and after letting it soak for awhile, I scape off the stubborn bits with my thumbnail or anything else that will not damage the paint.

Warm water and decal set or solvent should losen them up enough to do the trick.

Sometimes water will loosen them back up, or some decal solvent will soften them back up to where you can pull it off.

Any tips for removing the stubborn 10% of old decals that don't seem to come off with tape?

New decals are obviously a must here, but you can likely remove the damaged ones with tape. Just burnish it down to remove all traces, then apply new decals as needed. No need to sand down to the bare plastic, repaint, regloss, etc...