Body filler vs. fiberglass
Both body filler and fibreglass are used in creating, smoothing and fixing car surfaces. The two materials overlap, but also have distinct areas of use. Body fillers often have fibreglass materials in them, but fibreglass often requires the use of body fillers to be properly finished.
In general, fibreglass is a coating used for the entire vehicle or object, while body filler is used to touch up or correct mistakes.
Body filler is a type of resin that comes in small cans or tubes and is used to coat over existing surfaces. It cannot replace paint, epoxy, or fibreglass, but is used to correct mistakes made with these other substances and smooth out specific areas to give a better texture.
Fibreglass is a multi-stage coating applied to a surface that requires some type of backing, a mat, and a mixture of resin and hardener. Many qualities of fibreglass depend on what ratio the hardener and resin are mixed and the qualities of both. The mat is either dipped in the fibreglass solution, or the solution is brushed on and allowed to dry, then carefully sanded down to remove rough edges.
Types of Body Fillers
Two different types of body fillers exist. The first type has fibreglass strands in it and is used to strengthen parts and objects in a similar manner to fibreglass, albeit over a much small area. This also allows the body filler to be used to touch up fibreglass work and correct areas that need to be made level or filled in. Short and long strand fibreglass fillers are available, depending on the type of work. The second type of body filler is the kind without fibreglass and is used only for light touch up or spot correction work; these do not provide strength or stability.
Both fibreglass and filler resin use the same type of compounds to harden and dry, but the fibreglass has the addition of a hardener and will often harden fully in one day, although it make take several days for it to completely cure. Filler can be used with hardener, but it is more commonly used alone and will take longer to properly cure. Generally, the warmer the temperature, the better both filler and fibreglass will cure, with the optimal temperature around 23.9 degrees Celsius.
Body Filler on Epoxy
While fibreglass cannot be used on any other material, fibreglass filler can be used on other coatings, but it is generally advised to use it on bare metal. If there is any type of epoxy coating, the filler will not bond properly to it and will have only a mechanical adhesion, which can lead to problems later on. Bare metal or fibreglass is best, and epoxy primers will only interfere with the filler.