18 year old lacquer job is showing microscopic cracks. Salt air and dew (salt included) are getting through to the metal. "surface" rust area under the paint is much larger than the rust that shows on the top. Tried sanding off the paint and primer. The rust is extremely hard and the die grinder with a wire brush only seems to polish the rust. There is slight pitting in the rusted areas. Any suggestion on how to treat the areas before repainting. Grinding, blasting, sanding or chemicals. What should work? Thanks for the help.