Let us know how far a pot of plastikote goes! I reckon it might take one and a half to cover the floor with a coat - at a guess. Maybe less - it does spread well. Yes I meant about 12" up the sides - maybe 6" would be fine though
. It might depend how much bedding you're having - I was thinking any sides under bedding level could be best to be done in case they pee at one side of the cage near the edge. Although that's unlikely. Its really just the floor that's important to waterproof really. Although the floor of my barred cage stays clean! With 6" bedding and a litter tray in the house and sand bath.
I think these things only get pee stained when there's not much bedding.
I probably wouldn't bother to paint the whole inside if it was me. But then if you ever wanted to re- use it for another hamster it would be easier to wipe down the painted surfaces.
Actually - easier would be - just paint the floor and 6 to 12" up the sides. Then if you ever want to reuse it for another hamster, then paint the whole inside! Because any hamster scent would wear off when aired a bit and painting after would seal it to prevent any scent.