Hi Dave,
I've done a fair amount of work with metalizers and may be able to help you. First, a question - did you prime the cowling before applying the aluminum? If not, get in the habit of doing so as it greatly improves the adherence of metal paints. My primer of choice is Alclad's Gray Microfiller and Primer which can be buffed out to a shine before applying the metal.
As for a protective gloss coat, Alclad Aqua Gloss is the thing. I've recently been experimenting with the new AK Xtreme Metal paints and am very impressed with them although Alclad is still my 1st choice for metal paints. Anyway, this photo shows a spare part, primed with Alclad Gray then sprayed with AK Aluminum. The forward half was then masked and a couple coats of Aqua Gloss airbrushed on to the back half. You literally can not tell the difference between the glossed and unglossed sections once the Aqua Gloss has dried.

Repeated attempts to get any of the paint to lift with masking tape failed to produce even a speck.
Hope this helps.
Cheers,
Eric