Hey John, If you are gonna rattle can it, AT LEAST spring for some gray or red Scotch Brite pads and some GOOD rattle can self etching primer. SEM makes a good one. We use it on structural parts that dont really show, and dont get topcaoted fully. It will make a big difference in how well that enamel bomb can paint will stick.
After you scrubby the whole thing (Scotch brite, dont push too hard if you buy red ones because they are nearly as agressive as 80 grit paper), put on some rubber gloves and blow it off while wiping with a paper towel (air only will not remove everything) and wipe it down with a wax remover and de-greaser product. If you dont wanna buy wax and de-greaser you can wipe it down with damp paper towels (To remove water soluble contaminents), then wipe it down with Windex or rubbing alcohol on a paper towel ( to remover petroleum based contaminants).
Then blast away with a medium coat of the etching primer. Be careful it will run easily.
If you put the color on right after the etching primer flashes (10 minutes or so depending on temp. and humidity, by the time you finish priming the place you started should be plenty ready for the color coat),you wont have to sand the etching primer. If you wait more than a couple hours, you'll have to lightly sand it, and reclean it.
Different primers act VERY differently, so if you use another brand, READ THE CAN.
This is FAR from the best way to prep , but most effective and quickest for a cheap and easy paint job. It actually holds better than automotive paint out of a gun. Too much film build with and too much overspray , even with an HVLP jamb gun. I learned the hard way.
And its a bitch trying to keep it all wet without leaning into it while spraying. 
Larry
Rattle-canning is cheaper, easier, and looks just as good.