An idea...

[mmayo]

I occasionally need to fill small cracks or gaps with wood putty. If smeared on the piece it takes forever to sand away the excess. This is how I eliminate the excess and leave only a tiny 1/16" strip to sand. Blue tape placed in neat rows on each side of the defect.

Now I can really press the wood putty into the crack.

After a minute or so remove the tape slowly.

That tiny line of putty is easy to sand.

 

[mmayo]

Another photo of a vertical line of filler - hard to see, but easy to sand in a few strokes.

 

[KenV]

Putty knife is traditional, but I find a plastic motel key card or similar works better and is easier to clean.

Good techniqie for limiting the mess. Also works to limit scorch marks with pyrography edges.