I have 4 aluminum columns on the front porch of my home that are about 20 years old and an off white colorThe are just kind of dated and a couple spots have scratches and suchI wanted to know if there is a way to paint over the factory finish and what I need to do so.
Primers for metal fall into two categories, ferrous and non-ferrous90% of the time the metal getting painted is ferrous and the primer is red oxideAluminium is non-ferrous and needs a primer like called an etching primer, hammerite make one for priming aluminium before using hammerite, but you could follow on with any paint you wantIf your columns are not bare aluminium, but have the white factory paint finish then they do not need primingThe lightest of rub downs with a P120 or 180 grit paper (see how it goes) to key the surfaceAnd then use exterior oil-based paints as for woodwork- or if you want you could try car spray paints (or others)Upvc cleaners might bring up the current surface to an acceptable whiteness without painting.
