ALCLAD sells a clear "sealer," so i figured they expect their paint to be glossed over by some builders. That said, there are some who say that once you apply ANYTHING clear over metal paint that you ruin the finish and defeat the whole purpose of using stuff like ALCLAD in the first place.
I don't buy that argument. i use good old Future floor polish shot at 8-10 psi straight from the bottle. Lay it on lightly at first, then add more. I have gotten excellent results, with many metal projects, including the 1/32 Tamiya P-41 and the 1/32 Hobby Boss F-84, and decals adhere with little or no solvent. If you think the metal is now too shiny, knock it down with a LIGHT mist of Future with the air brush held at a distance from the plane while you spray, or you can go right to the heavy stuff with Testors flat.
You need to allow the Future to cure for a day or so, depending on humidity, before decaling so you don't wind up with finger prints. And, be very careful to apply any decal solvent over Future with great care. Do your best to confine the liquid to the decal itself, and NEVER put any on before you lay the decals down. Your clear finish will turn a cloudy white, and you'll have to re-shoot the section.
JA