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Fiberglass Filler: What Is It And Why Use It In Body Repairs?

What is fiberglass?

Raw fiberglass comes in a soft cloth like material. When saturated with liquid resin and hardened, it becomes hard and very strong. There are not too many fiberglass auto parts in today’s cars as they have all started using other composites like SMC and carbon fiber. However, fiberglass was found in early models of corvettes, truck tops, and many other parts. There are still replacement parts that are made from fiberglass and are still used for boats and jet skis.

The difference between fiberglass and fiberglass filler

The fiberglass filler comes in a can and is mixed with a cream hardener. It mixes in much the same way as regular body putty, but is thicker and a bit more difficult to mix. The filler actually has fiberglass mixed in. Comes short hair and long hair. This is the length of the fiberglass that is mixed into the filler. Both provide excellent waterproof properties as they do not absorb water. Both fiberglass fillers are stronger than normal body fillers. The long pile padding provides the greatest strength of the two. However, these fillers are very difficult to sand. The padding is also thick, which makes it difficult to level and smooth like normal body padding.

Why use fiberglass filler if it is so difficult to sand?

The reason we use fiberglass filler in auto body repair is not really the added strength, but the waterproof properties. It is recommended to apply a thin layer of fiberglass filler over any welding that is performed. Body filler absorbs moisture, which causes corrosion and rust. By using fiberglass, we eliminate the problem of moisture absorption. Since our primary goal is to seal the welded area, short-haired fiberglass is sufficient for the application.

What can fiberglass filler be used for?

This filler can be used on bare metal or fiberglass.

Finishing the repair

As I mentioned, fiberglass doesn’t sand well. That is why I recommend applying only a small amount to the welded areas and sanding them rough. Once this is done, you can apply auto body caulk on top of the fiberglass caulk and finish the repair as you normally would with auto body caulk.


You should always wear the proper protective gear when sanding any caulk. However, great care must be taken when sanding fiberglass products. Not only does it sting and irritate the skin, but fiberglass is extremely unhealthy to breathe in. Make sure to wear an approved dust respirator, gloves, eye protection, and you may even want to wear a disposable paint suit. If any of the fiberglass comes in contact with your skin, take a cold shower. This will help keep the pores small and allow the fiberglass to wash off.

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