Rustoleum is fine, but my personal preference for sandable primer is DupliColor light grey sandable. If I have something really rough/ nasty (I work with a lot of resin figures that tend to be on the rough side) I will use Rustoleum or Duplicolor high build formula. For my Star Wars weapons when I do a mod I will use the high build formula to lay on as thick a coat as possible and then smooth that down with a sanding sponge. Sometimes I will swtich to rust brown primer and lay on a smooth coat so I can see if I am "burning through" and sanding a high spot too much.
The walmart cheapie paints for a dollar or so are fine for topcoats; I prefer thier flat black Colorspot paint for all of my weapons due to its rather greenish tint. This contrasts perfectly with my other satin and semi gloss black colors that I use when building weapons. All of my weapons are primed, sanded, and then painted metallic grey, silver and brass. Two satin clear coats go on, then the final color coats. The color coats are gently sanded with wet steel wool to show the paint underneat. In time and with wear it will develop it's own natural wear pattern.