So it's time to start sanding your project for the first time. If the surface is already painted then start with 250grit, then 400 grit, then 800grit (if the old paint is peeling, you may want to get an even lower grit sandpaper to sand down to the original surface, you don't want your fresh paint peeling because the old paint is still peeling under it). If the surface is not already then just go with the 400grit then the 800grit. You don't need to use a lot of pressure with the final grit sandpaper. At this point you're going for a smooth surface but rough enough to give the primer something to grip on to (it should pretty much feel smooth to the touch). Once you're done sanding, clean your project very well. The sanding process will leave a lot of dust that you DO NOT want under the primer. Then give your project some time to dry (a dry cloth can be used to speed up the drying process, but still give it some time to dry).
With the sanding finished, it's primer time. I have only ever used 1 type of primer and it has never let me down. Rustoleum Automotive Primer. It comes in a few shades of greys so I usually pick one that is quite a bit different than the color I'm spraying so I can be sure I cover every square inch with paint.
You've sanded your project, you've got your primer, and you've got your handle attached. Now please don't go spraying where ever "feels" right and in crazy patterns. There is a technique to laying down the paint and it's important you get the hang of it to ensure an even finish. This technique is to start your spray off of the piece you're painting, go over the piece in 1 straight line, and end your spray once you're past the piece you're painting. Then repeat this for the next spray, slightly overlapping your last spray. Remember to start your first spray at the top, bottom, far left, or far right of the piece and work your way to the opposite end. Look at this picture below for a visual on how this works.
Please excuse my excellent MS Paint skills, but you can see how each spray should start and end off of the piece being painted and each spray should slightly overlap the previous spray to ensure good coverage. Also notice that the first spray starts at the far right and each spray after moves towards the left.
So you know the technique now, it's really not that hard. Just stick with it until all of your project is primered. I recommend 2-3 coats of primer because you will be sanding it (not excessively though).
You should have started out with something like this:
And now you should have something like this:
(although this is only after 1 coat of primer)
If you got any runs in the primer, do not mess with them. You'll take care of them when you sand the primer. Trying to get rid of a run while the primer is still wet is just going to cause you do to more work because I can assure you that you can't do anything about that run until you're able to sand. If you're getting multiple runs, it means you're laying the primer on too thick and you need to speed up your sprays. Move the can a little faster and the paint will go on a little thinner.
Give your primer a couple of hours to dry. It will probably be dry enough to work with after an hour, but you need patience to paint. If you try to start sanding and the paint or primer ends up still being wet under the surface, you're going to make 10x more work for yourself. So turn on the tv, wash your car, grab a few beers, spend some time with the kids, take a nap, jerk off. I don't care what you do, just give the primer a few hours to dry.
Once the primer is dry, you get to sand again. But this time you'll be wet sanding. Grab your 400grit sandpaper, 800grit sandpaper, spray bottle (full of water, ONLY WATER), and sanding block if you chose to buy one. Choose a section of your project to start on and spray it with water (give it a few squirts) and wet your sandpaper. Start sanding. Be sort of gentle with your wet sanding. You're not trying to sand all the primer off, nor are you trying to leave deep grooves. You're trying to smooth out the primer so that your paint will have a nice smooth appearance when you get to that point. Do not try to push harder or sand faster to save time. Get into a nice steady motion, apply constant pressure (but not too much), and occasionally spray some more water on your project. Water is your lubrication that will keep the sandpaper flowing nicely and not pulling off too much primer. As you should always do, start with your lowest grit sandpaper (400 in this case) and end with your highest grit sandpaper (800 in this case).