How to Remove Diesel Exhaust Stains From Gelcoat

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When a boat builder lays up a fiberglass hull, a mold release agent is sprayed into the boat mold, then a thin layer of Gel Coat is sprayed over the mold, followed by fiberglass, a core and more fiberglass. That thin Gel Coat -- the first layer applied -- stays on the fiberglass and provides all the color and gloss. If diesel exhaust stains develop on the surface of the Gel Coat, the right cleaner takes them off. If the stain goes deeper, a bit of elbow grease and a product from the automotive world can help.

Items you will need

  • Fiberglass-stain absorbent gel

  • Rags

  • Water

  • Garden hose

  • Disposable paintbrush

  • Polishing compound

  • 7-inch polisher with wood bonnet

  • Wax or Gel Coat sealer

Apply the fiberglass stain remover to the stained area with a clean rag. The stain is the result of soot being deposited on your hull. Soot is the gray-black "smoke" that comes from all diesel engines. Wet exhaust systems mix the exhaust gases with seawater and discharge those gases from ports on the side of the hull. Dry exhaust systems discharge the soot directly into the atmosphere and, inevitably, some stays with your boat.

Wait 10 minutes. If the stain is simply laying on the surface of the Gel Coat, the stain will begin to loosen, perhaps even run down the side of the boat.

Wipe or rinse the stain remover off of your hull and look at the results. If the stain remover almost worked, repeat the application.

Inspect the area once more. If the stain is still present, dampen the disposable paintbrush, then use the dampened brush to apply a thin coat of polishing compound to the stained Gel Coat. Use the 7-inch polisher to spread the polish and get the stain out.

Wax the formerly-stained area, or apply a Gel Coat sealer to the area.


  • This project involves working with both tools and chemicals. Appropriate precautions, including those recommended by the manufacturers, should be followed.


  • There are several stain-absorbent gels for removing stains from fiberglass. Your marina or fellow boaters will have experience using most of them and should be asked about their experiences with them.
  • Preventative maintenance -- cleaning the soot from diesel exhaust systems off of the Gel Coat before it builds up -- is the best cure for this kind of stain.
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