Norman,

Here is what I do to fill in gaps. I take a roll of masking tape and mask off either side of the crack. I then take some wood puddy and work it into the crack with a puddy knife. You can remove the tape immediately or after it drys. I usually add saw dust to my puddy to get a match with the surrounding wood. This will take a little experimentation to get a match. I like to mask off the surrounding wood because I find that, if I don't, I get discoloration when I wet out the wood.

Regards,

Jack