If you just spray primer on ali then yes it more or less just full off, it's basicly like a non stick pan. One way to deal with ali is to prep it throughly with a body shop DA or random orbit sander with a very fine grit pad. You need to make the whole surface uniformaly flat, this is a not a job you want to do by hand. You could use a jitter bug or sheet sander but they are hard work compared to a DA sander. Next step would be to use etch primer flat off then start with colour then clear.

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Fine Scotchbrite (green) then highbuild etch, use stopper on any imperfections and paint/laquer.

I have just polished a piece of ally sheet as a test to see if it will hold laquer and if prepared properly it can be pressure washed with no ill effects.