I like to rock a thick oil based paint for the frame. You can cover anything, it levels nice and looks great. Solid and won't come off easy. Put painters tape on the surface of rail, don't paint top or the corners if square. If round just run a strip down the main part you'll slide.
Some of the places I've worked that rock spray paint on frames also chip or scratch off easily. If you want to paint the top just use a light spray paint. That will scratch off on the corners and some of the top depending on how much you ride it. If I had a backyard setup I'd throw some spray on the top personally but doesn't really matter.
If you can wirebrush it down to more bare metal that'll help it slide better if it doesn't have any shit on it. If you really wanna make it shiny bust out the flap as well, just don't put grooves in it.