If you're prepping them to prime/ paint them, i wouldnt use the power sander. All you need to do is dull the finish enough so that the primer has something to grab on to. And you don't want sanding scratches to show through. 150- 220 would work fine- i would use a sponge with fine grit.
Use a bonding primer- I'm liking Zinnser 123+ ( same as Smart Prime) , or reg 123
Use a bonding primer- I'm liking Zinnser 123+ ( same as Smart Prime) , or reg 123
