Wood filler, also know as wood putty, is an excellent substance used for repairing cracks and holes in wood. It is fairly simple to use, and, depending on the type you choose, quite easy to clean up. Wood filler is invaluable for restoring your wood to a state as strong and smooth as it was previously, and it is also very good at filling and matching wood grain so you can restain your items accurately and evenly. There are two different types of wood filler: water-based, and solvent-based, and which type you use ultimately comes down to personal preference.
Water-Based Wood Filler
It used to be that solvent-based wood filler was the only type available. As tools were improved and chemical technology was advanced, water-based wood fillers were introduced. These are nearly identical to solvent-based fillers in utility. They serve to fill holes and repair wood as well as solvent-based fillers, and they match up to wood grain to let you stain your wood items again. The primary difference between the two types of fillers is in the clean-up. Water-based fillers, by virtue of how they are made, can be cleaned using only water. The user must take care to clean it up before it dries, however, because it is nearly impossible to get off your tools once it has dried.
Beyond clean-up, water-based wood fillers have a number of other benefits as well. Since they do not contain harsh chemicals and solvents, they are much better for the environment, and also much healthier to work with. You do not have to worry about the fumes coming off, and there is no risk if you get it on your skin, although you want to clean it off as soon as possible since you don't want it drying on you.
Solvent-Based Wood Filler
Solvent-based wood fillers have been around for quite a long time. Up until relatively recently, it was all you could get. As water-based fillers have gained popularity, solvent-based fillers are used less and less. Solvent-based fillers have a chemical base and require that you use turpentine or acetone to clean them up. They also emit fumes that can be dangerous if you breathe too much of them. Solvent-based fillers take a longer time to dry than water-based fillers.
Filling Holes or Smoothing Pores
On top of the different substances used to make wood filler, there are also a couple of different kinds with varying consistency to use for different jobs. For filling cracks and holes, you want a thicker wood filler. These usually are a putty used to push into the cavity and smooth out the top. For smaller jobs, such as smoothing out the pores in wood, you need a thinner filler that is more easily spread over a large surface area. In either case, you want to make sure the filler is completely dry before you sand or stain it.