Epoxy fillers to mix with epoxy resin to thicken it. For most boatbuilding projects, a combination of silica powder and wood flour blend is all that is required in the way of fillers.
Epoxy fillers are sold by the litre.
When mixed into epoxy, white glass balloons produce a surface that can easily be sanded to a smooth finish before painting. Mix it with epoxy to form a thick paste and apply it to any rough surfaces. After curing, the layer can be sanded smooth. The trick is to sand most of it off so that the filler is only filling the holes. For a perfect finish the cycle can be repeated several times. It is possible to paint directly onto the sanded surface.
Because glass balloons make the epoxy layer porous, they are not recommended for below the waterline.
Micro-fibres give a greater tensile strength than other fillers.
Unlike glass balloons, phenolic micro-balloons are not hollow, so they can be used below the waterline.
Mix silica powder with epoxy when laminating two pieces of wood together. This thickens the epoxy slightly so that it is sufficient to bridge any small gaps between the wood. It produces a white mixture.
Wood flour blend is a strong filler that makes very strong fillets. By using in conjunction with silica powder the colour of the fillet can be adjusted to match the wood being filleted.