I have used Krylon Crystal Clear over my homemade decals with success. I have also used Microscale Decal Bonded and the Testors that have come in the decal kit. I like the spray can best because the decals tend to run when you brush anything on them.
I have had this for several years. It is expensive but to me worth it. I use it to punch gauge decals and baremetal foil. Makes it a lot easier to put round decals inside the dash. I also have a larger set I got at Harbor Freight.