Hey there, I personally wouldn't recommend painting the bike tank that way. First you have the decal problem and they don't put much clear on to begin with. You run the risk of sanding through the clear and into the base coat. Then you'll have to paint the whole thing anyway. If you feel the need to try it that way,. I would use a wet paper in 600 grit and none coarser. That will give enough for the coatings to grip to without getting heavy sand scratches. Good luck!