Repairing wood defects with filler is a multi-step process. Here’s how to do it right:

1. Clean the Defect: Remove any loose wood fibers, old finish, or debris from the hole or crack. A clean surface ensures good adhesion.
2. Choose the Right Filler: Select a filler based on the defect and finish. For stained projects, use a stainable filler. For large structural voids, a two-part epoxy is best. For nail holes under paint, a simple wood putty is sufficient.
3. Apply the Filler: Use a putty knife to press the filler firmly into the defect. Overfill the area slightly, as most fillers will shrink as they dry. This is known as filling "proud."
4. Allow for Full Drying: Let the filler dry completely according to the manufacturer's instructions. This can range from minutes for putty to hours for epoxy.
5. Sand Flush: Once fully cured, sand the filler smooth and flush with the surrounding wood surface. Start with a medium-grit sandpaper (e.g., 150) and progress to a finer grit (e.g., 220) that matches your final sanding grit for the rest of the project.

• Wood Filler: A paste-like substance used to fill imperfections in wood. It typically hardens and can be sanded, stained, and finished. Often water-based.
• Wood Putty: A non-hardening, pliable substance, typically oil-based, used for filling small holes and cracks in finished or painted wood. It is not meant to be sanded.
• Epoxy Filler: A two-part filler consisting of a resin and a hardener. It creates an extremely strong, durable, and waterproof repair suitable for large or structural defects.
• Stainable Filler: A type of wood filler designed to absorb stain, allowing it to blend in with the surrounding wood. Results can vary, so testing is crucial.
• Proud: An woodworking term meaning to be intentionally raised slightly above the surrounding surface, with the intention of being sanded flush later.

"The biggest challenge with wood filler is color matching, especially under a clear finish. No filler will ever look exactly like real wood grain. The trick is to get close and then use other techniques to hide the repair. Sometimes, the best approach for a stained project is to stain the wood first, then use a color-matched putty to fill nail holes. For larger defects, I often mix sawdust from the same project into a clear epoxy. This creates a custom filler that matches the wood species perfectly. Always test your entire finishing schedule on a scrap piece with a filled defect to see how it will look." - Professional Furniture Restorer

Filling Nail Holes in Painted Baseboards: You have 50 small nail holes in pine baseboards that will be painted white.
The calculator would recommend a simple, fast-drying wood putty or spackle. Since it will be painted, color match is irrelevant. The focus is on a smooth, sandable repair.

Repairing a Large Knot-Hole in a Rustic Oak Tabletop: You have a 1-inch diameter knot-hole in an oak slab that will have a clear finish.
The calculator would strongly recommend a two-part epoxy. You could tint the epoxy black for a modern, decorative look, or mix it with oak sawdust to create a more natural-looking plug. A standard filler would likely shrink and crack in such a large void.

• Using the Wrong Type of Filler: Using a non-hardening putty on a surface that needs to be sanded, or a non-stainable filler on a stained project.
• Underfilling the Defect: Most fillers shrink as they dry, so not overfilling slightly will result in a noticeable divot.
• Sanding Before It's Fully Cured: This can tear the filler out of the hole or clog your sandpaper.
• Unrealistic Color Matching Expectations: Expecting a filler to perfectly mimic wood grain. A good repair is inconspicuous, but rarely truly invisible under a clear finish.

• Furniture Restoration: Repairing scratches, gouges, and other damage on old furniture.
• Trim and Millwork: Filling nail holes from brad nailers in baseboards, door casings, and crown molding.
• Hardwood Floor Repair: Filling cracks or gaps between floorboards.
• Stabilizing Knots: Using epoxy to fill and stabilize loose knots in rustic-style lumber to make them a decorative feature.