How to Use Wood Filler: Grain Filler vs Gap Filler vs Epoxy

The word "filler" covers three very different products in woodworking, and using the wrong type is a common mistake. Grain filler fills the open pores of the wood to create a glass-smooth surface for finishing. Gap filler (wood putty) fills holes, cracks, dents and defects. Epoxy filler fills large voids, structural gaps and deep damage. Here is when to use each and how to apply them properly.

Grain filler

Grain filler is essential when finishing open-grained woods like oak, ash, mahogany and walnut with a film-forming finish (lacquer, varnish or shellac). Without it, the finish sinks into the pores over time, creating a textured, uneven surface. Grain filler is a thick paste that is worked into the wood pores, allowed to set, then scraped and sanded back to leave a perfectly smooth surface with filled pores.

Apply grain filler across the grain with a stiff brush or squeegee, forcing it into the pores. Let it set until it hazes (typically 15-30 minutes depending on the product), then scrape the excess off across the grain with a plastic scraper or a piece of stiff card. Do not scrape with the grain — you will pull the filler back out of the pores. Once scraped clean, let it dry fully (overnight is safest), then sand lightly with 320 grit to remove any remaining surface residue.

Grain filler is available in neutral (for use under opaque finishes) and tinted versions (to match or contrast with the wood colour). A contrasting filler — dark filler in light oak, for example — accentuates the grain pattern dramatically. See our thixotropic grain filler guide and which woods need grain filler for more detail.

Gap filler (wood putty)

Gap filler is for visible defects — nail holes, screw holes, small cracks, dents, chipped edges and joint gaps. It is typically a stiff, dough-like material that is pressed into the defect, left to harden, then sanded flush. Water-based fillers (like Brummer interior filler) are easy to work with, accept stain, and sand cleanly. Solvent-based fillers are harder and more durable but less accepting of stain.

For best results, slightly overfill the defect and sand back after curing — fillers shrink slightly as they dry. On stained or dyed work, test the filler on a scrap piece first to check colour match — some fillers change colour significantly under stain. For critical colour-matching, mix sawdust from the project wood with PVA glue or CA adhesive to make a custom filler that matches the surrounding timber exactly.

Epoxy filler

Two-part epoxy is the choice for large voids, deep knot holes, structural cracks and any repair where strength matters as much as appearance. Mixed with pigment or wood dust, epoxy can be colour-matched to the surrounding wood. Mixed with clear dyes, it can create striking translucent fills in natural-edge pieces. Epoxy does not shrink, which makes it the only filler suitable for deep fills in a single application.

Apply epoxy to the void, slightly overfilling, and let it cure for 24 hours before sanding flush. Epoxy sands well but clogs sandpaper quickly — use fresh paper and go through the grits. It does not accept stain (the surface is non-porous), so if staining the surrounding wood, apply the stain first, mask the stained area, then fill with tinted epoxy colour-matched to the stained wood.