I'd skip the sand paper....you're going to be there forever. Drop them off to be sandblasted (or media blasted). It'll be a much better finish and way more consistent.
If they're powdercoated already, get some aircraft coating remover. BE CAREFUL!! That stuff will take your skin off....get a good pair of heavy PVC chemical resistant gloves. Put it on and let it sit for a bit. If you can, put them some place warm (ie, in the sun in an enclosed container) as this will help the remover lift the powdercoating off faster. Rinse following the directions, usually clean water. Then sandblast them.
I suggest powder coat...but if you want to paint them yourself, talk with some paint guys and follow their advice when it comes to primers and etc. Remember, 90% of the work is in the prep!