VEVOR 4x4 Post Base 4 Pcs, Inner Size 3.6"x3.6" Post Base Brackets, Heavy Duty Powder-Coated Post Anchor Matte Black Wood Post Brackets for Pavilion Deck Railing Support Deck Base Plate

They came powder coated black. Painting over them with Rustoleum just gives them better rust resistance.

Yes. But because these are factory powder coated, perhaps scuff them with a fine sandpaper first.

I wouldn’t see why they couldn’t. They would need some prep, but sure.

I installed my factory painted black supports a year go. They haven't discolored yet and still look new. I leave mine alone.

Yes. Scuff with red scotch brite pad and paint

I’m not sure. I used 4x4 pressure treated and they go in snug.

3.6 inches

Read the description. It tells you what the inside diameter is

If you insert a 4x4 post into ,the slot i would say there is about a quarter inch of play so that you are able to adjust the post to exact ally where you want it before screwing to the base.

3.6x3.6

Our's are all filled with 4x4s so I can't accurately measure the inside for you but they're really close to the 4x4" size. They're absolutely fantastic. I'll measure them tonight when I get home from work!