It is important to know what you are doing when you prepare to sand paint off any surface, especially fiberglass. Unlike wood, fiberglass is much easier to scar and scuff and thus needs to be treated a bit differently. So, how do you sand the paint off of fiberglass?
Step 1 - Select the Appropriate Equipment
The most important part of sanding fiberglass is choosing the right equipment. Utilizing an orbital sander will save a lot of time and effort. A soft-head pad will be helpful as well. Begin with a medium grit sandpaper (100 grit is best). Make sure you have safety equipment, goggles, and a dust mask or a painter's mask. An orbital sander would be good for sanding a flat piece, like a surfboard, or any flat area.
For something with a lot of curved surfaces, like a fiberglass boat, the use of an orbital sander is very limited. The painter may have to do a lot of hand sanding. A D/A sander is a better choice. A D/A has a somewhat flexible round pad where you put your sandpaper. The sandpaper has adhesive on one side, and is cut to fit the D/A sander. The D/A is an air tool requiring an air compressor.
Step 2 - Prepare the Area
Place a drop cloth on the floor in the area you are going to be sanding in. Cover anything else you want to protect from the dust produced.
Step 3 - Begin Sanding
Begin lightly and slowly until you get the feel for the orbital sander's movement. Make sure you use the dust mask and safety goggles to protect yourself. After you hit the high spots and delaminated areas you need to check for any holes or spaces that need to be filled. Fill them in with a fiberglass putty from the local hardware store. Follow the instructions for applying the putty.
TIP: Ed Kimble, our painting expert, strongly suggest that you "try not to dig into any curved surfaces of the fiberglass with the sander."
Step 4 - Second Round
Now you will have to wait. The putty has to have time to dry and cure before you work with it. Once the putty has had time to cure use the sander a second time. This will help sand the holes flush with the rest of the fiberglass surface.
Step 4 - Up the Ante
Once you have finished a second sanding, it is time to increase the grit of the sandpaper to super fine, around 1,000 grit. This super fine grit will smooth and polish the laminate on the fiberglass. Begin slowly until you are used to the feel of the orbital sander with the new sandpaper on it.
Step 5 - Finish
Finally, clean the fiberglass using a soft cloth and acetone. Make sure you are in a well-ventilated area when you use the acetone to avoid being overwhelmed by the fumes. Now the fiberglass is ready to be painted again or repaired.
TIP: Ed also notes that you should "wash the fiberglass down with water after you are all done sanding and cleaning with acetone."
Trim the sandpaper to the size of the sander.
Always use goggles and a mask.
Don't apply too much pressure as you sand. It is very easy to sand directly through the fiberglass.
Using liquid fiberglass sander is another option, although much messier and it is harder to control the outcome. If you do choose to use a liquid sander, be very careful with it, using gloves and protective gear to prevent an injury.