Are they regular (waterslide) decals or dry transfers?

If they are plain decals, the best way is to gloss coat the area you want to cover, then apply with a setting solution like Microsol. If it needs more work to conform, like over seams or welds, use Microset on top of that. The gloss clear on top to seal. That's the best way to get the painted on look. After sealing, you can weather however you like, nothing will disturb the decal.

If they are dry transfers, just apply and then clear coat.