Metalizer should be sprayed onto bare plastic. It's a very fragile paint in my experience, you can't mask over it with tape. The plastic underneath also needs to be polished out so scratches in the plastic won't show up in the paint.
Mask off and paint the other colors first, like the anti-glare panels, and nose/tail markings, then mask them off and paint the Metalizer. If you used the "buffable" Metalizer, you can buff to a shine when the paint dries.
Testors makes a sealer for the Metalizer to improve the durability, but it's not necessary if you're really careful about handling it (cotton gloves work well). You won't have any difficulties in applying decals either way.
Don't let me scare you away from it, it looks good when applied, just give it some TLC. You may want to practice with it on an old kit in order to get used to how it works.
Glenn