i'm in the process of converting a caravan chassis into a box-van trailer cum workshop shed. its an old chassis, not galvanised and with a fair bit of surface rust. nothing terminal, however. its all 3mm c- section, except for the main front-back beams which have been enclosed by a piece of 3mm plate, for a length of about 6ft. as far as i can see, the inside of this "box" is as rusty as the outside. is there a clever way of derusting it, prior to coating with rustproofer? i'm going to derust the accessible bits with a wire brush, then paint the lot - I' m thinking a high zinc primer, and then lathering with underseal.- its likely to spend a lot of time being dragged across grass fields for car boot sales.