How to Use Wood Filler

1. Mix the Wood Filler

   Stir the wood filler in its container with a putty knife. Water-based wood filler may not need to be stirred, but solvent-based wood putties usually need a thorough stirring as components will separate. Mix the material until it is a smooth, peanut-butter-like consistency, with a uniform color and texture.

2. 
   Spread the Wood Filler

   Press the wood filler firmly and deep into the crack, then scrape off the excess, making sure not to gouge the knife into the patched area.

   Petroleum-based putty hardens quickly, so you'll need to work fast. By contrast, water-based products are creamy and stay wet much longer.
   

3. Smooth the Filler (Optional)

   Press the wood filler deeper into the crack with your finger, if necessary, then wipe off the excess from the wood. To remove the filler from your finger, quickly wipe it off with a dry cloth. If any residue remains, you can remove it with water (for water-based products), or mineral spirits (for petroleum-based wood putty). 

   Scrape the filler from your putty knife onto a scrap of wood, then remove any residue from the blade using a rag and water, or mineral spirits. 

4. Sand the Wood Filler Smooth

   Sand by hand after the wood filler is completely dry and hard, using 220-grit sandpaper. Move the sandpaper in the same direction as the wood grain. For large filled areas, use an oscillating sander, starting with medium-grit sandpaper and following with fine 220-grit or 320-grit paper.

5. Wipe With a Tack Cloth

   Wipe down the wood with a tack cloth, using light pressure, to remove sawdust and filler debris.

6. Finish the Wood

   Stain or paint the wood. Once the area is clean, the filled crack or patch is ready for a protective finish. Staining the project will help to equalize the color differences between the filler and the surrounding wood, but don't expect the filler to be entirely invisible. Because wood filler absorbs stain differently than unfinished wood, the color is rarely a perfect match. 

Tips For Working With Wood Filler

Check Wood Filler Condition

Discard the wood filler if parts of the filler are dry, if the filler doesn't mix up well, or if there is a rancid smell, indicating contamination.

Don't Overdo the Filler

Be conservative when you apply the filler to the joint or crack with the putty knife; you can always apply more if it shrinks slightly as it dries.

Remove Major Dust With Vacuum

If you have a huge amount of dust, use a shop vacuum to remove the bulk of the dust, then use a tack cloth to remove the fine dust film.

Avoid Heavy Pressure on the Tack Cloth

A tack cloth is a sticky, wax-coated cheesecloth that is designed to remove fine dust from wood surfaces before finishing. Pressing too hard will embed the wood with the tack cloth's wax, necessitating another round of sanding.

Use Pre-colored Wood Fillers

Expert woodworkers sometimes pre-color the wood filler before application in order to make it blend in better with the wood finish. Wood stain can be mixed into the wet filler before it is applied. You may need to use a darker stain when tinting the filler to match colors. Use water-based stains if using a water-based wood filler. Use solvent-based stains if using a petroleum-based wood filler.