When connecting two pieces of wood together the joint is not always as nice as you would like. It would be nice to do perfect work but you can't throw out a piece just because a joint is less than perfect. Small cracks and gaps can be filled with a simple technique called Glue Sand. This technique is widely used by woodworkers who make the less the perfect hand cut dovetail joints.
This photo shows the end of a cantilever bench that I was building. Note that there are small gaps.
Step 1: Apply Glue
Glue in spread in the area where the gaps need to be filled. I put on more glue than necessary to show that is it okay. It will be wiped off.
Step 2: Wipe Away Glue
Quickly wipe away the excess glue. The glue will stay in the gaps. Unfortunately the end grain also absorbs some. Do not clean up with water. Just wipe away with a towel and move to sanding.
Step 3: Sand
Immediately start sanding. The generated dust will mix with the glue and fill the gap. Start with 100 grit and keep sanding till there is no more trace of glue in the end grain. Then move to 150 grit and finally 220 grit. Note in the photo that no glue is to be seen in the end grain and, when compared to the photo at the start, the joint is greatly improved in appearance.