Finishing those panels is a lot like doing automotive painting. The area you plan to paint will need to be sanded for it to stick, architectural coatings are baked on, very tough, formulated to repel dirt, and not real receptive to paints. You'll need to apply a primer (solvent based metal primer is the best), followed by a good quality paint. Acrylic paint (not latex) works, urethane is better, epoxy is best.
Mask so when you sand you're not sanding beyond where paint will be laid, fine sandpaper (320 or finer) to give the new finish something to bite to. You don't want to sand anything outside the area you're painting either, for appearance, and you don't want the original coating to have an area where it will absorb moisture and fail. You'll probably have to re mask depending on what you use (sanding will damage the mask edge and give rough edges, or allow your paint to bleed). Apply primer, when the primer is set, apply at least two coats of paint & remove the masking. Pretty much the same process as painting stripes on a vehicle. If you don't prep it properly, it'll peel. Use a cheap paint and it'll fade & fail quick because there is no shade to protect it from the sun.
For the amount of labor involved, prefab letters or vinyl would be quicker and more economical, and be able to be quickly and easily replaced as they age, rather than going through the process of repainting again. For wood, brick, block, concrete, painting is pretty straightforward, not so much for these specialty coated substrates.
Good luck to you, if you do tackle it, post some pics.
