Slow down. There is no need to sand blast or even sand all the paint off. Paint sticks to paint just fine if there is some "scuffing" or very light scratching of the existing paint. First make sure you have the seat post height adjusted to your liking, including the allen adjustment underneath and any elastomer changes. You want to know the exact height it will be permanently mounted. Mark that height. You will paint everything above that but now below where it clamps. Use some tape to tape off the area below your mark. Get some 600 or maybe even 400 grit sand paper. Dip the paper in water mixed with a light liquid soap and sand the upper portion. You don't have to get all the paint off, just get some light scratches. Dry it off by setting it in the sun, or if you have a compressor blow out all the nooks and crannies. This will simulate the spray can somewhat. You don't want the spray can blowing water out while you paint. Everything must be bone dry. Rub it with a clean cloth to remove dust. Now you're read y to paint.

Tips on painting with spray cans.

Put the paint can in the sun for at least 15 minutes this time of year to warm up the paint. Keep the part in the sun until you are ready to paint. Spray it in two light coats so you don't get any runs.

I've never actually painted a seatpost but I have a 38 year old Falcon Convertible that has needed plenty of parts painted over the years!