A gelcoat surface is a very smooth, mirror-like finish when it is new. As gelcoat ages, it becomes porous which means the more easily it stains, the worse it looks and the harder it is to clean. Most all gelcoat can be restored, providing it is not completely worn through. Exhaust stains, like diesel, can develop on the surface of the Gel Coat. These stains can set in on the wax surface of the gel coat but the right cleaner can remove them. If the stain goes deeper, a bit of elbow grease will be needed.
Clean and inspect the surface with a wash with a mild cleaner like 3M’s boat soap or some alternative stronger cleaners if needed. After cleaning, dry and inspect surface for needs to be addressed: scratches, dings, cracking and crazing and to fix these flaws before rubbing out the gelcoat and restoring its shine. See the fiberglass/gelcoat repair section for options and the West System Video for “how to” instruction.
Apply a fiberglass stain remover, like 3M Fiberglass Restorer and Wax , Mothers Marine Black Streak Remover, or 3M Marine Cleaner and Wax to the stained gelcoat area with a clean rag. The black/gray stains are the result of soot being deposited on your hull. It can happen when wet exhaust systems mix the exhaust gases with seawater and discharge those gases or dry exhaust systems discharging the soot directly into the atmosphere and it lands on your boat.
These liquid one-step products that combine a polish or compound with a wax remove oxidation, and protect the gelcoat in one step.
Wait 10-15 minutes. If the stain is simply laying on the surface of the Gel Coat, the stain will begin to loosen; rinse and inspect the results.
If the stain is lifting, repeat. If not, then something stronger like a more abrasive compound with a polisher may be needed; in that case a wax will be needed to restore the shine. Each time compound products are used, some of the gelcoat is removed. Use caution to use the least aggressive product that will get the job done. Frequently all that may be needed is to apply a protective coating of wax, such as 3M Ultra Performance Wax, Scotchgard Marine Wax, or Mothers Marine Synthetic Wax to help keep that shine.