Wood Fillers (copy right handyman magazine)
There are two kinds of wood fillers — hardening and non-hardening — It depends on what you need to fill and the finish you’re using.
If the project is going to be stained, then fill the nail holes with non-hardening filler, such as Color Putty, after the staining and before applying the top coat. But if the project is going to be painted, then use a hardening filler like Wood Dough (DAP) or Plastic Wood (Boyle-Midway).
NON-HARDENING FILLER. The advantage to the non-hardening filler is it’s easy to match color. Because Color Putty comes in 17 shades, it’s fairly easy to match it to a project that has been stained. And that makes the filled areas nearly invisible. And since this type of filler doesn’t harden, you don’t have to wait overnight to apply the topcoat of finish. Another thing you might like about non-hardening filler is its long shelf life. It doesn’t harden in the container. And if it gets too dry to work with, you can just add a couple drops of boiled linseed oil or mineral spirits to soften it.
HARDENING FILLER. They’re perfect for when you have a hole, a gap, or a knot that’s too large or deep for the soft non-hardening filler. Or whenever a project is going to be painted.
When using a hardening filler, fill the holes before sanding the project. Then wait overnight to allow the filler to dry, and sand the wood and filler smooth.
The biggest drawback to hardening fillers is they don’t accept stain like wood does. When stained, these fillers tend to stand out from the surrounding wood. But regardless of the filler you use, it’s a good idea to first test it on a piece of scrap wood that’s finished the same way as your project. This way you don’t get any nasty surprises.
For wood that I’m going to paint, I like to use Bondo, drys hard & fast, does not shrink. Nail holes & cracks can be sanded & painted within 15 minutes. Knot holes & build up for sculpting will take a little longer.
Of course the best filler is the same species wood with matching grain ,other alternatives are epoxy with some matching color mixed in ,great for loose knots after you remove any bark that might be present.You can many times add an inlay over a defect or damage to hide it,making it into a design element. I’ve even been able to use iron on banding a time or two.