How to prepare aluminum columns on front porch for repainting?

https://www.hunker.com/13416999/how-to-paint-aluminum-columns

Wash them with a water-based/oil-free soap. Rinse and dry the columns before applying a metal etching primer to the columns. A polyester brush might give you the smoothest finish. Let the primer dry-- about 3-4 hours-- then paint your columns.

lightly sand making sure to remove any loose paint, I prefer Rust-oleume

If they are already painted I would remove any loose paint and then wash them down with a 1/2 warm water and 1/2 white vinegar solution! Let dry completely before painting! The paint will not stick to dirty surfaces!

Wash well, let dry, use a good primer, then paint. Use paint formulas intended for metal.