I had the same rusting problem with my 76 NYB. My area of rust was along the lower trunk edge. I had it repaired with metal welded in but now 2 years later it is starting to rust through again. Bondo is a short term used car way of fixing these problems. Sadly rust is a fact of life with these old cars and the price we pay for our romance with them. Also have similar problem with my 81 IMP. Love em or leave em.
Bob Becker 76NYB, 81 IMP hot on the trail of an 82 IMP