On these, I have not painted one yet. I have found that a coat of Pledge Floor Gloss or whatever it's called ( used to be Future) gives a slicker surface for the decals. But the PFG will get sticky if it gets too wet.
I use Micro Sol (? The red bottle).
I have used a hair dryer in the past as well as hot water on a rag pressed onto the decals. On this one, I didn't have to use any heat. Just micro Sol several times and press the decals down a few times till they stay down.
I use the micro Sol because I have it on hand and it's always worked if a solvent is going to. And my bottle is pretty old.