There are a number of different thickeners, fillers, and additives that serve many different purposes. Most fillers will work in polyester, vinyl ester, or epoxy. The resin choice will depend on the application. Always remember that filling gaps or voids with plain resin can make the resin brittle, and doing so is very wasteful compared to filling with a thickened resin. Blending thickeners and fillers to achieve a particular working or cured property is very common. When using polyester or vinyl ester resin, mix the filler before adding the catalyst; when using epoxy, always mix the resin and hardener together completely before adding any fillers or other additives. Always check for that your filler is compatible with your specific resin prior to mixing.
Milled Glass Fibers: Milled glass fibers (chopped glass), are small, cut pieces of fiberglass. These fibers are used as a structural filler, usually when strength is needed. It is not recommended to use milled glass fibers to thicken a resin, or if the cured resin would have to be sanded, because it becomes very hard to sand. Without an additional thickener such as silica added, the epoxy mixture also would not have very much body and would sag.
Silica: Also known as colloidal silica, this filler is best for thickening a resin when adhesive properties are most important, but also to thicken it for purposes of holding a shape, or preventing resin from running out of an area. For some applications, only a little bit of silica is used; in others, silica is added until the resin is as thick as peanut butter. This thickened mixture can also be used for vertical or filleting applications. Though it is not as hard as milled glass fibers, a cured resin thickened with silica is still difficult to sand after curing.
Microballoons: Microballoons are small spheres that are designed for lighter weight fairing, and for making cured resin easy to sand. Microballoons can also be used to thicken a resin for vertical application, but do not have the adhesive or strength properties of silica or milled glass fibers. Generally, microballoons can be glass or phenolic in composition.
Wood Flour: Wood flour is also known as wood dust. It is commonly used for aesthetic purposes to create a desired appearance. It can be used as an adhesive thickener for filleting or other bonding applications, and can be added to reach a concentration that won't sag during vertical applications. The dust from different wood species will have different tonal variations.
Graphite Dust: Graphite dust is designed as an additive that will turn a resin system black in color, and leave a low-friction surface for applications such as bearing surfaces.
Colorants and Dyes: Always check to make sure that the colorants you're intending to use are compatible with the chosen resin type. There are specific tints and dyes that are alcohol based, gel based, or powder based, and all are used to create various special effects. Many of these colorants and additives are very concentrated and a little will go a long way—add only a little bit at a time until you reach the desired saturation.