My favorite wood filler is not a wood filler at all, but an auto body filler called Bondo. It's a two part putty that, when mixed with a hardener, allows it to set and harden.
Scoop some out onto a cardboard surface that was intended for trash so that it can be thrown away after. (I have a putty knife I use exclusively for bondo because they don't ever get completely clean).
Then add a little of the hardener and mix.
Once the hardener is added, you need to work quickly because the putty sets very rapidly. The more hardener you add the faster it sets.
Bondo is perfect for building up edges or corners of furniture and filling large dents as it dries harder than regular wood filler. It is also my go-to when filling hardware holes.
Lay it on with your putty knife and try to smooth it as much as possible before it sets. Sometimes, like in the picture below, your bondo will harden before you're done smoothing it out. If this happens, don't worry; it will still sand out or you can float out the edges with glazing putty (explained below).
Sand it down with a 120 grit sandpaper and reapply if needed. (Note - Do NOT apply Bondo too heavily, as it can be tough to sand down.)
When I really need a smooth, glass-like finish, I used another Bondo product called glazing putty. You don't have to mix it, it goes on smooth, sands very, very easily and the finish is AMAZING.
