The white dust is Zinc Hydroxide, or the byproduct of corroded galvanising. It can appear on either electroplated galvanised sheet metal like the lockers, or on hot dip galvanising like the roll bar. Zinc Hydroxide won't hold a paint coat and will continue to corrode. You need to wash it back with a weak ammonia-based rinse (really weak, using household-laundry grade ammonia), and scrub it off with a stiff bristle brush. Unfortunately, this oxidation will be happening under other areas of paint nearby that are not yet peeling, so to stop it you will need to remove the paint back a couple of inches around the affected area. The affected area will no longer have any galvanised protection, so you need to think of how you intend to prep the bare steel to prevent good old regular rust. Provided you have removed all of the Zinc Hydroxide, those cold-gal type paints can work well as they are rich in zinc and tie into the sound galvanising around the affected area. If money was no object, then sandblasting and re galvanising would bring it back like new (obviously!).
