I know several "well known and established" modelers who use Scalecoat paint on their brass, and then bake the piece in the oven. They claim that hardens the paint and makes a super glossy finish for water slide decals.
Others use Future Floor Wax, or the latest version called "Pledge with Futur Shine"
I, on the other hand, am a great fan of Testors High Gloss Clearcoat Spray # 2936. Spray on two light mists, then a final "Wet Coat." Let it cure fully (about a day) and then decal. The advantages of this Clearcote are:
1) It produces an almost mirror-like surface. See below
You can clearly see see the track and ties reflected in the surface
2) The Clearcoat Gloss is impervious to any decal solvent. Including Walthers Solvaset, MicroSol, Micro Set, etc
3) It has a cutting matt type resiliance to fine cuts from a #11 blade, so its very easy to slit silvered decals, apply setting solution. The cuts will never be seen.
4) It goes over any type of paint without crazing
5) It takes any type of top coat, without crazing
6) it really hides the decal film
The only downside is the final wet coat needs a level surface to prevent runs or sags. So you need to paint one surface, let it sit for few hours, then paint the next. You can't put the painted surface on anything for about 24 hours, so you need some sort of holder (rotisserie) to paint a car in a reasonable time. See the cabin car photo above.
But the results are worth it:
The decal film is vitrually invisible:
