For boat building and repairs, epoxy's superior strength makes it ideal for filling larger structural gaps. But filling gaps and voids with epoxy requires thickening the epoxy to a peanut butter consistency to hold the desired form.
You can either make your own thickened epoxy by blending the epoxy/resin mixture with a thickening agent (filler), such as colloidal silica, so it can bridge large gaps in structural applications without loss of strength. Or you can use a pre-thickened, non-sagging mixed epoxy product packaged in a convenient cartridge, such as West System Six10 and TotalBoat Thixo (regular formula, not Thixo Fast Cure).
There are a lot of epoxy fillers available, and different fillers are used for different purposes. Colloidal silica products (e.g., West System 406 and TotalBoat Silica Thickener) blended with mixed epoxy will create the strongest non-sagging structural gap filling adhesive. This type of gap-filling adhesive is great for structural applications such as attaching stringers, ribs, and other parts that are below the waterline and out of sight. Other fillers, such as microballoons and milled glass fiber are used alone (and in combination) for different purposes—from structural to cosmetic—as shown below.
|STRUCTURAL||SAG RESISTANCE / STRUCTURAL||COSMETIC / SANDABILITY|
|Milled Glass Fiber||Colloidal Silica & Milled Glass Fiber||Colloidal Silica||Colloidal Silica & Microballoons||Microballoons*|
|High strength & low sag resistance||Good strength & sag resistance||High strength & greatest sag resistance||Good strength and some sandability||Least strength & best sandability|
*Note that phenolic microballoons are a little easier to sand than glass microballoons.