Any chemical stripper will take all coatings off, leaving you with a bare aluminum frame. With no topcoat, the aluminum will oxidize, turning a mottled sort of gray color... and that's about it. It's not attractive, but it's not like the frame is going to up and disintegrate either. That gray oxidization layer is akin to rust on ferrous materials, it provides a protective layer-- that will transfer onto anything that rubs against it. So unless you want to hand-polish the frame every couple of weeks, easier just to paint/powdercoat/clearcoat it and be done with it. Also, most aluminum frames that come painted aren't exceptionally attractive to begin with. Paint hides.