When I touch the sides of the camper it leaves black marks

Bought a used MPG 184 last year. It has fiberglass sides and I just power washed it, hand washed it, and put a coat of NuFinish on it.
Any time we touch the sides of the camper it leaves a black streak (see pic from just dragging fingers over the side). We can wash the marks off easy or use a Magic Eraser and it comes off, but when we touch it again, it leaves more black marks. Any ideas on why this is happening and how to fix it?
 

Attachments

  • IMG_4616.JPG
    IMG_4616.JPG
    949.3 KB · Views: 11
Top