NMF and Future

Anytime you overcoat with a clear coat you change the look. If you applied a matte sheen aluminum, it will now take on a glossier look. I would spray an old fuel tank and see what you think, is it acceptable to you or not. Then decide whether or not you will apply it to your project.

As Gerald said, clear coating any NMF will alter the appearance.  With Alclad polished Aluminum, you should be able to decal without a gloss coat.  Alclad is a lacquer, so you can use an acrylic wash to define panel line detail without a topcoat.  There are some risks involved however, so the advice to try first on a scrap piece is good advice.

Regards,  Rick