airbrushing a spoiler

[carborush]

I'm airbrushing a protoform body spoiler
the protectionfilm is on the bottom backside of the spoiler
wich would mean painting it on the topside
how can I finish of the paint as a final protective layer ?

i use lots of fascolor white paint and faskoat now for the final layers on the inside of the shell body
will faskoat on top make the paint look dull ?