Defects happen
With any project there will be defects to fix, for a variety of reasons. With the batch of drawers assembled there were a few that had some gaps in the dovetails. It appears that these were mostly due to cupping of the face boards due to the rapid humidity change. Here in Wisconsin, we were getting our first cold weather over the last month and this meant a drastic drop in humidity. Some of the drawer fronts cupped unnoticed prior to cutting the dovetails. This lead to some gaps to fill .
When I was young, and learning woodworking from my father, he would say “A craftsman knows how to cover his tracks.” Often for small dings and gaps he would use elmers glue and sawdust. However these fixes had to wait a day to proceed with further work and often had nasty splotching when finish was applied. Now we have better choices of adhesives but wood as the primary filler, whether as a powder, thin strips or veneer is still the primary component.
When fixing small defects like these, as well as corner chips, I typically use thin cyanoacrylate adhesive (super glue or CA). The bottle of glue and squirt bottle of accelerator are always at hand when sanding, planing and scraping the parts near the end of a project.
Filler materials
In filling the gaps, it is handy to have wood flour also known as sanding dust on hand. I have multiple colors of wood flour that I have collected over the years, notably: White ash, white oak, okoume, mahogany, cherry, walnut. Each is a match to woods I have used. Here are a few of the choices on hand. The finest are from sanding, coarser are from the band saw. To collect a given color based on your current project, clean the air filter of the shop vac well and then attach the vac to your sander. With a random orbit sander you will have enough for many projects after wearing out a few disks sanding. Now you just tap the sawdust off the filter onto a piece of cardboard or into your container.
Adjusting the color
When using wood flour, the material will often be a bit darker than the background wood. This is most noticeable with epoxies which also will add their own amber or reddish tint. To lighten the color, use some high density silica powder such as West Systems 404. I often will use 1/3 as much as the wood flour to get a decent match. If you only need a bit of lightening colloidal silica such as West Systems 406 will work. The wood flour and high density silica was used on almost all of the fillets on the inside of the boat: http://bronkalla.com/Interior_floor.htm . White coloring in the epoxy was used for the contrasting deck stripes: http://bronkalla.com/decking_fiberglass.htm
When you need something a bit darker than the wood flour on hand, powdered “earth pigments” provide the easiest answer. I have a set of powdered pigments that I got from Lee Valley a number of years ago but these no longer seem to be available. The powdered pigments work well whether using super glue or epoxy. I have heard of others using powdered tempera or milk paints as colorants but have not tried them myself. When using epoxy, then you have added options of: Universal Tinting Colors (UTCs) such as Tints-all, Mixol or Transtint dyes. Unfortunately the Tints-all tubes crumble / fracture after a few years.
Small and tiny gaps
For tiny gaps you have 2 primary choices. Ignore them and then fill after finishing with colored wax or use the sanding dust and CA method. For the sanding dust and CA, apply a small amount of CA into the gaps and then start sanding with your random orbit sander. The sanding dust will naturally fill in the gap and the glue will harden in the power. You may have to do this a few times. If you are overzealous in your glue application you will ruing the sanding paper in short order. If there are mixed or contrasting woods, I will often sand from the lighter wood towards the darker.
Above is an example of some tear out that would qualify as a small gap. Certainly too small to insert a piece of wood as filler.
The wood flour has been applied. Ready for the CA.
Now the CA glue has been applied. I have tried to keep the overflow on the side plywood rather than the face endgrain as much as possible.
At first, the CA and wood dust mixture will seem dark. However, it is approximately the same color the surrounding wood will darken to once coated with a clear finish.
Medium gaps
Medium gaps (1/16″ to 1/8″) can be filled with solid wood, wood flour and glue or epoxy. It varies with the background and your schedule. Here are a couple of examples.
Sawing a solid wood filler. Wedge shaped pieces offer a better opportuity for a tight fit than straight. I like cutting shallow crescents out of matching stock as this gives a nice taper and 2 filler pieces from each cut. Remember you must visualize this from the standpoint of the end grain of the piece rather than the face. 
The cut does not need to be very shallow. I am using a 1/4″ 3 TPI blade in this photo. Yes, the blade guard is way too high but it makes for a better photo.
Here is the piece inserted into the gap in the drawer and the glue applied on the plywood side. Adding the glue on the face side makes for too much CA in the end grain and potential splotches when finishing.
Cutting it off with a zero tooth set Japanese saw. These are GREAT for cutting off not only filler pieces but also bungs for screws and many other uses. No scratching of the surface.
The completed repair. Nearly invisible. If I had dug a bit deeper in the scrap bin I could have yet a better match. (yes I have a split to fix to the right of it.)
Large gaps, knots and splits
This is really a category of its own, but with tinted and filled epoxy you can have spectacular results.
This really falls into more categories. Those that are irregular and need a liquid filler or those that are small and straight and can be patched with a matching piece of wood. I will focus on epoxy filling. The epoxy is mixed and tinted to match the gaps appropriately. Blue masking tape works well for dams to prevent the epoxy from running out. Careful application of heat with a propane torch brings the bubbles to the surface . However practice first on scrap!
Yes it is a big project when you have to crawl on it to sand it.
And who says finishing is not fun?
Here is the rustic ash table top (7 feet long) that had knots and cracks filled with tinted epoxy. It nearly filled my shop as you can see tools nearby on both sides. Great fun!
