Well, I have been custom painting T-Formers for some time now (you can check out my website if you wish:
http://www.shadowcmndr.blogspot.com). I only use Acrylic paints. They dry very fast (within minutes) and they are water-based and so after painting, spray some windex in a small cup, put your brushes in it for about 1 minute, then rinse with warm water. Cleans them all right up and works like a charm.
Enamel paint on the other hand is rather difficult to clean and takes for ever to dry (24 hours sometimes). Plus you have to use paint thinner to clean up afterward.
If you don't want rubbing or chipping...thee
ONLY way you're going to prevent rubs and chips is
PREPARATION. You must prepare your transformer before you paint it. So, here's what you need to do.
1) Clean the figure. Use soap and water, wash it up, then let it air dry for a day.
2) Sand it down. Use 600 grit sandpaper to rough up the plastic surfaces (ALL of the surfaces). You may take it apart first, then sand. Taking it apart will result in the best sanding and painting.
3) After you sand it, prime it with black or grey primer. Use Krylon flat black or Krylon grey primer for this. Do light sprays (too much paint will cause rubs). Keep in mind, you already sanded it first so you really don't need to add too much primer.
4) After priming and after it is dried, transform the figure at least 2-3 times. You will start to see rubs appear and paint chipping on the primer. This is good. It is good because now you know where to focus your sanding on. So, start sanding again. When done sanding again, prime just those spots that rubbed with a little bit of primer (not too much).
5) Now that you repaired the rub spots, you can start painting. If you use a brush (remember thin coats) and if you spray paint (also do thin coats). The ideal tool to use is an airbrush (you can get a good set up for about $125 (brush and air tank). Having an airbrush is a MASSIVE difference between spray painting out of a can. It's really worth buying an airbrush.
6) If you don't have an airbrush, just remember thin coats.
7) Once the paint applications are done, let the figure dry for about 3-5 days.
THEN, carefully transform it (avoid fast transformation and forcing parts - it will rub (even if prepared properly) if you force parts together). Once you transformed it, transform it back. You will see that either your sanding paid off and no rubs!!
OR you will have to do some more light sanding and touching up.
When done, spray it with a sealer of some sort if you wish. I don't seal mine anymore because if you prepare it right, you won't have to seal it.
9) Once you have completely finished painting your figure, let it fully dry for 2 weeks before regular use or posing. Acrylics fully cure after several days and the difference of cured acrylics vs lightly cured acrylics is quite big.
10) I use Tamiya or Model Master. Games Workshop is good too. * Acrylics all the way!