on a similar note (well related to MOTs anyway!)....
if a car has an advisary last MOT about some rust... then on the next MOT the part appears all fixed, but covered with underseal, is this likely to cause problems? i ask as i had a few advisaries on the last MOT on the SL (not when i owned it), but i'm restoring it so have fixed quite a few things. I heard that they often fail it if they cannot see the bare weld etc.......