The key to removing scratches from fiberglass is to know whether the project is one you can successfully tackle yourself. While it's laudable to approach small repairs in your bathtub or shower with enthusiasm, it also pays to be long on realism. Such is the balance you should aim for when you assess scratched fiberglass. Two low-risk removal methods will give you a good start.
Clean the fiberglass with soapy water and a sponge so that you can get a good look at the scratch. The depth – not the length – will provide a good idea of the work ahead. If the scratch is a shallow surface scratch, you should be able to remove it yourself. A deeper scratch – or one that that you can easily wedge a fingernail into – might require the expertise of a fiberglass refinisher.
Dip a rag into a jar of petroleum jelly and rub the scratch in a circular motion. Expect to repeat the application many times before you see a noticeable difference in the appearance of the scratch.
Apply a small amount of polishing compound to a soft cloth if the petroleum jelly fails to remove the scratch. Rub the compound into the scratch in a circular motion. Rub your finger over the scratch to assess your progress. Repeat this process several times, or until the scratch disappears.
Wipe away the compound with a rag. Apply a coat of fiberglass wax to the surface, unless the location would jeopardize personal safety, such as in the standing area of a fiberglass shower. Follow the directions on the wax to seal the repair properly.