In this Autos, Motorcycles & Planes video tutorial you will learn how to apply fiberglass filler to a welded area. Fiberglass or aluminum filler should always be added to an area that is welded. This will ensure proper sealing and prevent moisture ingress. There are two types of fiberglass fillers; long haired and short haired. Long haired is stronger than short haired filler. First you got to mix the filler and hardener together as per directions on the containers. Wear rubber gloves and a putty knife when you do this. This can take a few minutes to thoroughly mix. Then apply the mixture on the welded area and smooth the filler out so it is level with the surrounding area.