There are three setting levels for wood fillers: initial set, mid set and hard set. These hardness levels depend on the time given to the drying process and the temperature of the environment. Typically, filler hardens quicker in warm temperatures. A sealer should only be applied over wood filler once the filler has completely dried.
Types of Filler
There are two different kinds of wood filler. Wood putty is used for small wood repairs, such as nicks and nail holes. This filler has a single composition, it dries quickly and it will be ready for sealing in approximately two hours. Wood epoxy has a two-part composition, designed to be as strong as the wood itself. Epoxy filler is used on large wood repairs, such as chewed furniture or to repair dry rot, and requires at least 24 hours to dry before sealing.
Initial set for epoxy filler can occur in 10 minutes for temperatures above 32.2 degrees C, and in 30 minutes at 21.1 degrees C. This initial set should not be sealed as it is still soft and can easily be broken. The purpose of this set is to allow the filler to conform to the shape of the object and to blend in with the rest of the wood.
After about two hours of drying, the epoxy wood filler will be mid set. The outside layer of the filler has now completely dried and feels as hard as the wood. During mid set, it is possible to sand the filler so that it is smooth, but sealing should still not begin.
After 24 to 48 hours, depending on how many separate coats of epoxy you apply, the filler will have had time to dry all the way to the surface of the wood and be hard set. If you have multiple coats of filler, wait 48 hours. With just one coat, you can seal the filler after 24 hours.