The first step is to dissolve any rust on the metal with navel jelly. Just brush it on with a super cheap brush (one of those throw away brushes) and wait 15-20 minutes. Then, then wipe off the dissolved rust with paper towels. Let the metal dry completely.
I recommend using a brush for your metal painting project, don't spray paint unless you're crazy - drips, overspray, bad for the environment, not worth it! Also, wear knee pads; you'll thank me later!
Next sand the entire piece you are working on with 320 grit metal sandpaper and a power hand sander, while wearing a mask.
DEWALT Electric Sander, 1/4 Sheet, 14, 000 OPM (DWE6411)
Next, use primer and paint over all previously rusty spots. Let the primer dry overnight. You can buy a mid-priced brush that will fit inside a gallon paint can (or you can saw it shorter if it doesn't). Don't use a cheap one because it will lose hairs and be a huge pain in the neck. Buy a new empty 1 gallon paint can at places like OSH and fill it up to 1/3 with mineral spirits and just put the brush in it overnight with the lid on.
1 Quart Bulls Eye 1-2-3 Primer Sealer Stain Killer
Wipe your brush and reuse it for painting, then clean the brush in the same bucket of mineral spirits with paper towels when you're done painting. This saves you from having to clean it twice and also keeps the brush more pliable for painting later. Dispose the can at the neighborhood hazard materials drive, or drop off center when the job is done.
Try and paint on a cloudier day rather than a sunny one. It can be very hard to see where you've painted in bright sunlight. Also, it's better for the paint if it doesn't go on in direct sunlight. If you have a lawn umbrella, you could use it as shade over your work.
Rust-Oleum 7777730 Stops Rust Brush On Paint, Half Pint, Pick Color
Tape down newspaper (its much easier to deal with than plastic and you can match tight corners much easier). It's best to tape it down, because even if it's not windy when you start, I guarantee it will get windy before you finish! You can tape around mountings on brick/concrete - or freehand around the mountings if you're feeling steady. If you are painting over brick and it happens to drip, you can blot the spot then scrape the brick with a screwdriver, it will disappear. This works somewhat on concrete also, but not as well. Please don't rely on this trick completely however!
Use rustoleum satin or semi-gloss, and plan on doing two coats even if you're painting things the same color they were before (usually, the old coat is faded so much it's like a different color and will show through). I personally think semi-gloss holds up better against weather over time, but you can do either one. Please use a mask with organic vapor inserts. I know you're outside, but you're still getting a lot of exposure to these brain-warping chemicals.
Start on one end of the job, go through and paint on all of the metal, then return to same starting point and do the second coat in same order so paint has even drying time and enough time to dry between coats.
After cleaning your brush, you can spread out the paper towels in sun so they "outgas" before you throw them away and are less flammable (I always worry about spontaneous cumbustion in the trash can. I know, its probably ridiculous!).
I did the same process on the chairs.
Here are some finished pictures of a rusty metal job I did. I should have tqken "before" pictures!