Press these putty-like fillers into cracks in wood up to 1/8" wide. At full strength, they can be sanded, painted, and varnished.

Nitrocellulose fillers contain real wood fibers in a solvent base, so they look and act like wood.

Epoxy fillers come in a stick with a resin on the outside and a hardener on the inside. Cut the desired amount from the sticks, knead to a uniform color to mix, and press into place.