The use of filler is voluntary. Stain works fine without it as do varnish, polyurethane, shellac and lacquer. Filler is an aid to a glassy, consistent finish. It fills and flattens pores in wood. Proper use of filler requires specific drying times.
Wood has different pore structures that impart different characteristics to different species. Maple, birch, cherry, poplar and alder have tight grains and typically don't need fillers. Wood such as oak, ash, mahogany, walnut and fir have open pores and are more likely to benefit from wood filler.
Guitars and Pianos
When filler -- sometimes referred to as paste filler -- is added to porous wood, it closes and seals the pores, creating a uniform, consistently flat surface for the subsequent application of stain. Think of a musical instrument finish such as a piano or guitar with a glassy, hard finish.
Traditional paste filler is oil-based. More contemporary paste fillers are water-based. Fillers consist of a formula containing three main ingredients: a bulking agent, binder, and carrier or solvent. The bulking agent is the actual filler. The bulking agent most often is gypsum, sand or silica, which fills and blocks the pores. The binder is a resin, which is the oil-base -- the part of the formula that requires the drying time -- and the solvent or carrier, which is mineral spirits for oil-based and water for water-based.
Oil or Water
Before you choose between a water-based or oil-based filler, decide what type of top-coat product you plan on using. If you plan on using a water-based top coat, use a water-based filler. If you plan on using an oil-based top coat, use an oil-based filler.
Oil-Base Dry Times
Haze Over Time
After the initial application of oil-based filler, wait between five and 20 minutes for it to haze over. Drying times vary depending on temperature and humidity. When it becomes dull, wipe off the excess with a stiff cloth; burlap works well for this.
Allow the filler to cure for 12 hours and examine the surface. If you are not satisfied with the results, repeat the initial application.
Allow the filler to cure or dry for at least 48 hours when you are satisfied with the initial application. Check the surface by sanding it lightly with 320-grit sandpaper. The sandpaper should function normally. If it gums up or sticks, or the finish is obviously not dry, it may take a week or more to completely cure depending on humidity and temperature. If it's below 50 degrees Fahrenheit, it's not going to dry fast.
Water-Based Dry Times
After the initial application of water-based filler, don't wait for it to haze over. Wipe it off immediately after applying it. Wait one hour and test sand it by hand with 320-grit sandpaper. If the paper gums up, it isn't dry. It should produce a fine, white powder. Wait another hour, sand it again and repeat if necessary. Stain can be added at this point but ideally, you should wait overnight.
Stain if Desired
The application of stain is optional. It can be applied before or after filling. Apply it before filling to bring out grain patterns. The application of stain after it's filled often results in an opaque appearance because the stain won't absorb into filled wood at the same rate as unfilled wood.