Short answer... Rattle Can.
Ridiculously long answer....
Powder peels off. It just doesn't bite like paint. Thats because its a powder "Coating" its a plastic coating. Also the metal can rust under the coating. And it will. Trust me.
Any metal that has mill scale (very hard dark layer) on it will be much more difficult to prep. I used a dozen 80 grit sanding pads on my DA for my skid plates. The sliders were better.
If you have mill scale to deal with, 50-80 grit. The amount of work to remove the mill scale isn't worth the effort unless you have a sandblaster at your disposal.
If its true bare metal, 120 grit will leave sanding marks in it. (thats how you tell the difference)
So you want to clean it first, get the oil off the metal before sanding. Cause if you sand first, you're grinding the oil into the grooves you make, which is harder to clean off.
So Thinner/ Cleaner/ Degreaser first. Clean it really good.
Then sand however you plan on sanding (follow the reccomendations of your paint)
Clean it again when you're done.
Next I used Rustoleum Self etching primer on top of the sanded metal. 2-3 coats.
Once dry scuff the self etch with 320 lightly. You don't want to remove it, just scuff it.
I wipe it off with a damp cloth, to remove the dust.
Then paint with whatever you want paint it with. I used Rustoleum Professional Gloss Black.
One thing I don't understand with the bedliner crowd... If your slider has any texture, its going to hold dirt and mud etc. I guess if your rig is strictly a trail toy I get it, but I daily my truck and I don't want mud and stuff on my pants getting in and out.
Just food for thought.
FWIW, I spent hours and hours reading forums and threads looking for info on coating sliders and skids. The general response is just paint them. And don't powdercoat anything thats going to get drug on rocks.
IMO, the best coating is POR15, but if you buy the prep + coating + top coat etc, the cost gets pretty steep. The prep and dry time is kind of a PITA too. I think paint does just as good of a job on something that's going to get beat up... cause I plan on beating my sliders up.