Grain filling is one of the most debated topics in guitar finishing. Some builders do it every time. Others skip it entirely. Here is how to decide what your build needs, and how to do it if you choose to fill.
What is grain filling?
Open-grain woods like mahogany, ash, and walnut have visible pores across the surface. If you apply finish directly, the finish sinks into those pores as it cures, creating a slightly textured surface. Even after many coats, the pore texture may show as small depressions in a gloss finish.
Grain filling closes those pores before the finish goes on, so the finish lays flat from the first coat.
Do you need to grain fill?
It depends on the wood and the finish you want.
Mahogany bodies: yes, if you want a glass-flat gloss finish. The pores are large and very visible. No fill = ripple texture in gloss finishes.
Ash bodies: yes for gloss finishes. The grain lines are dramatic and show significantly under gloss.
Basswood, alder: barely open grain. Usually no fill needed. A couple of sealer coats is sufficient.
Maple tops (solid maple, not veneer): very fine grain. Fill is optional for most finishes.
If you are doing a satin or matte finish, grain filling matters less: the texture of the finish itself hides minor pore texture. For a mirror-flat gloss, fill is necessary on open-grain woods.
The three grain filler options
Paste grain filler (oil-based): the traditional approach. Apply across the grain with a stiff brush or piece of burlap, working it into the pores. Let it flash to a dull look (usually 20-30 minutes). Wipe across the grain to remove the excess. Let cure 24 hours minimum before sanding. Compatible with lacquer, varnish, and polyurethane topcoats. Not compatible with shellac or water-based finishes without testing.
Shellac (pumped in coats): apply several seal coats of shellac and sand back between each coat. This builds up in the pores and levels over several applications. Slower but more compatible with a wider range of topcoats. Works well under lacquer.
Water-based grain filler: newer option, dries fast, cleans up with water. Compatible with water-based finishes. Read the manufacturer’s data sheet for topcoat compatibility.
How to apply paste filler
- Sand the bare wood to 220. Remove all dust.
- Apply filler with a piece of burlap or stiff card, working it diagonally across the grain to pack it into the pores.
- Let it flash to a dull, hazy appearance. Do not let it cure completely hard before wiping.
- Wipe across the grain with burlap. The goal is to leave filler in the pores and remove it from the surface.
- Let it cure 24 hours. Sand lightly with 220 to remove any raised grain.
- Inspect under raking light. Unfilled pores appear as small shadows. Apply a second coat if needed.
What grain filling is not
Grain filler does not fix surface defects, gouges, or sanding scratches. Those need to be addressed before the filler goes on. Filler is for texture, not repair.
A well-filled body under a gloss finish is one of the most satisfying surfaces you can achieve on a guitar. The extra day of preparation is visible in the final result.