In some cases, when the floors are getting refinished, there are some gaps, cracks and nail holes, that  need to be taken care of. Well, some types of cracks can be filled and some should be left alone. Here is an explanation.

Things like regular nail holes, little cracks and gaps inside the pieces of wood can be filled pretty easy and the putty more than likely will stay. These holes can be filled when the floors are getting sanded (between the rough first sand, usually 36 grit, and fine, usually 80), before the final coat, or, in case something left behind, after the final coat. In the first case, I always use Zar red oak wood putty. Believe me, this filler blends with pretty much any wood type- oak, pine, fur – you name it. The only wood that requires a special colored putty is maple. 

Zar can be also used  before the final coat,obviously if it is a natural finish, although it will not blend in that good. Normally, if I missed something while I was sanding the floor and applied two coat of finish already, I use Minwax Golden Oak. It actually made for this purpose and can be used even after the final coat.

There are some types of the gaps that either cannot, or should not be filled.

1. You will not be able to do anything, if you have a space between the floor and the baseboard: since the level is different and in most cases the floor ends where the baseboard starts, the putty will just continue falling in the gap. In the end you will have much less product, dirty baseboard and the gap that is still there.  If you have this issue it is much better to just install  quarter-rounds.

2. Long spaces between the pieces of wood. This is usual for wide pine flooring, or just an old wood. The reason why you can’t take care of those using just the regular filler is because the floor moves: it expands and shrinks depending on humidity. So the regular putty simply won’t stay their.It will pop, you will step on it and leave a scratch on the floor. If I have to fill these gaps, I would use epoxy: it is much stronger. The minuses of this are: there is no color in it, so you will be able to see the epoxy clearly and it takes about 24 hrs to dry.