The major factory (AS, SH, etc) appear to lightly scuff the Al caps and fill them to contour with polyester filler and/or gel coat. Polyester resin systems stick very well to Al in my experience. West Systems sells a conversion primer for Al when applying epoxy.

If the filler layer is thick, many really repairman apply fiberglass, using polyester resin or polyester "gelcoat" to build thickness, prior to applying the topcoat system of choice.

This is a very common repair and I am aware of many instances where the cracks were scarfed out, filled with "bondo", sanded to contour, and top-coated. They have held up for more than a decade, tied down outside in SoCal.