I have a 1972 Chevy C-10 Pick-up. The filler material (seam sealer?) in the roof gutter channels is cracked and deteriorating. Additionally, rust is forming in the channels. What is this material? Should I completely remove the present filler and replace it with new or, is it ok to clean away the visible rust, coat the existing with some new material, and paint the cab? If it is to be replaced, what product should be used to closely match the original appearance? If it is to be coated, what product would serve to fill/ conceal the cracks and be compatible with the existing material and auto primers/ paints? Thanks.