What If... I'm painting aluminum or vinyl windows?
To prepare aluminum and vinyl windows, you must remove whatever will interfere with the bonding of the paint. Scrub off the oxidation from aluminum windows with a metallic scouring pad or 120-grit sanding sponge.
Vinyl windows might exhibit some oxidation, but the main problem will be their gloss finish. Remove both by scuff-sanding with 220-grit sandpaper or a sanding sponge.
Prime aluminum with a water-base corrosion-inhibiting primer. Paint both aluminum and vinyl with two coats of a high-quality acrylic latex exterior paint.