Advice Needed

I am going to paint the roof of my 72 D100. I am not doing a restoration but I want it to be white like it was originally. What do I do with the channels? I am thinking sand them, then POR 15 them, put a big bead of seam sealer, then paint. Please give me some advice. Lets "pretend" I don't know what I am doing.IMG_1809.JPGIMG_1812.JPG