if you are repainting something here is the way I do it.
To start off I pour a little 91% alcohol into a little jar, I use a lid to a salsa jar myself. Dip a brush into the alcohol and put the alcohol on the model where you want the paint to peel. The way I did this boxcar is I did one rib at a time. Once the alcohol is on the car, wait half a minute to a minute depending on the type of paint(acrylic, enamel). After the alcohol has been on the car for the desired ammount, take the alcohol off the car. You can either take it off by tilting the car and letting the alcohol come to the bottom or the way I do it is to take a little piece of cloth or something similar and very gently touch the cloth to the model. Be careful or you will take alot of paint off. Once the alcohol is off take a paint brush and again dip it in the alcohol, and wipe it on a towel so that the brush is wet, but not saturated. Decide where you want the paint to peel, and start "picking" at the paint with your brush. Depending on how long you left the alcohol on the car, the paint will either start to slowly come off, or the paint will come off in globs(my first experience ). You want to have the paint come off slowly, not in globs (If the paint is starting to come off in globs, leave that part of the car alone for awhile, to let the alcohol dry). Once the paint starts to come off slowly, (it starts to appear in little, very little rust pits) you have achieved the peeling paint effect.
On another note, I would use a brush that is not of much value to you. It separates, the bristles and the brush does not end up in very good shape.