I'm trying to come up with a method for verifying the straightness of body work on our 1970-72 Skylark GS', using an original non-NOS fender. Cross filing doesn't seem to be appropriate (file in one direction, then at 90 degrees from the original direction), as one direction will look good but the other isn't. Sanding a guide coat with a paint stick won't catch gradual indentations. Sanding with a 5" circular block has the same problem as cross filing. I'm thinking that the best we can do is use the paint stick method with guide coat. Then apply silver paint and visually inspect for any gradual imperfections?