There should be instructions on the cans. Follow them.
If there aren't, then assume that you need a primer coat. I have found that Tremclad works pretty well on metal without priming, but I've never tried it on aluminum. Engine enamal should work on anything, and is very temperature tolerant.
Paint doesn't stick well to aluminum, so a wide variety of processes has been developed in industry. A chromate finish used to be standard, but has fallen out of favor due to toxicity of the chemicals used. A common approach now is to chemically remove the oxide layer and then apply alodine or iridite. Paint will stick to these with a proper primer. You can take your parts to metal fab shop to have them alodined.
Otherwise just do what most people do: spray it with primer, then paint, and realize that the finish will easily rub or peel off..
Steel handles painting better than does aluminum. You have to remove any oxide or haze, let it dry, then apply the primer recommended for the probably-acrylic paint you're using.
You have to use primer. There is not enough binder in most paints, unelss they are expressly made for metal like some epoxy-base finishes.
Clean steel with a dilute mixture of sulphuric and phoshoric acids - this prevents, short term, flash oxidizing of the acid-treated steel. You must descale and clean any steel or iron sufrace prior to painting if you want a chance at long-term adhesion of the finish film.
Then apply the primer within a few hours, or oxide will begin to form. Meaning poor adhesion of the paint film. Follow the can labels scrupulously about recoat intervals or you'll also invite problems.
Keep the new finish out of sunlight for at least several days. This allows any trapped solvents to evaporate through finsish micropores. Otherwise the finish may blister.
Better paints for steel are the spray-on nitrocellouse lacquers/primers meant for automobile finishes. This is because the expansion of wood is not just thermal, it also repsonds to humidity in a big way, so finsihes meant for wood are stretchy, nitrocellulose lacquers are a little more brittle and resist film migration.
Do not use interior grade finishes on outside applications.