Whether you sand your fibreglass boat during the preparation for the winter lay up, or during the spring fitting out, sanding a fibreglass boat is a part of preparation for your fibreglass boat's new paint job. Like any paint job, the preparation, including sanding the old paint away, is the key to having a successful painting project and a paint job that will hold up well for more than one season.
Lay a large plastic sheet beneath your boat to catch any loose paint chips that may fall away when you are scraping the paint with a paint scraper. Remove loose or flaking paint, or bubbles in the paint job, by scraping the area with a 50-mm (2-inch) hook scraper.
Lay a strip of masking tape over the top of the waterline, so that the top edge of the tape is even with the top of the waterline. Use 80-grit sanding disk on your palm sander to remove the paint above the waterline.
Wash the sanded area above the waterline with mild liquid soap and water. Wipe the area down with a clean rag soaked in acetone and allow the area to dry.
Remove the tape from the waterline, apply masking tape so that the bottom edge of the masking tape is even with the top of the waterline and repeat the sanding process.
Wash the area below the waterline with mild soap and water, allow the area to dry and wipe down with rags soaked in acetone. Allow the area to dry before repainting.
Repaint the bottom of the boat below the waterline before you paint the area above the waterline.
Wear safety glasses and heavy rubber gloves while working with acetone. Acetone should be used in well-ventilated areas only. When you have finished with the acetone-soaked rags, deposit them in a covered metal rubbish bin.
Sanding dust from marine paint is difficult to contain. If your sander can be connected to your shop vacuum, it will help contain the dust. If the project is undertaken at the end of the season, you can throw a large sheet of plastic over the boat to help contain the dust. Wear a painter's respirator while sanding.