This is similar to common wood putty, but thinner, so it will fill in and smooth out woods such as mahogany, which is heavy and dense, but also porous. Wood fillers, which may also be called “pore fillers,” are typically composed of wood dust that’s combined with a binder and a diluent, which is basically a thinner. By filling in the porous surface of wood such as mahogany for use as a guitar body, it will be easier to stain and apply the final clear coat. Generally, the surface of the wood will be sanded prior to the application of the stain, and sometimes again before the clearcoat is applied. In some cases, the filler will shrink over time, so that when held at an angle to a bright light, the surface doesn’t appear perfectly smooth.