I've always used Testors Decal Bonder only because it's always worked for me so I've never tried anything else . It's sprayed on the decal sheet in 2 or 3 light coats and allowed to dry overnight , then the decals can be cut apart and applied like normal . It goes on very very thin so it doesn't affect how the decals look after they're applied . A lot of people just use a clear spay paint to do this but the Testors Decal Bonder goes on much thinner than any clear I've seen .