I don't use metalizer sealer for anything. If it's non-buffing metalizer and no weathering or masking is going to be done to it, I leave it bare. The non-buffing ones don't rub off like the buffing ones do. Aside from that, I cover MM metalizer with Testors clear coat lacquers that come in 1 oz jars for airbrushing. You can even use masking tape when coated with that. Pastel chalk won't stick to bare metalizer, but it will stick to semigloss or flat clear lacquer.
The buffing metalizers don't seem to be good for actually buffing them. They do shine up and all that, but they rub off. You can rub 'em off right down to the plastic easily. So, I just skip the buffing there & do a lacquer clear coat on those too. Anything that I want to shine like that is a job for Alclad II or Spaz-Stix, afterall.
MM enamel Chrome Silver 1790 is awesome.