There are many options for filling voids and defects, bad joints and more. Do they work? Should we?
I think I have invented one or two of my own and then had a thousand recipes from woodworkers telling me what they do to give the impression of a perfect dovetail or shoulder line to a tenon. The most common suggestion is glue and saw or sanding dust. This makes a filler but of course, even in the same wood, the colour will not match. In some places, a contrast will work acceptably well. I might go for a very deliberate contrast rather than a close or perfect match in some cases simply because I know that working on new wood a match can be good but over a few weeks the main body of wood will change colour in the light but the filler will not. In that case the filler becomes obviated by its inability to change.
I don’t really want to get into commercially made fillers but more of what works at the bench in the instancy of the moment so that I can get on with planing, paring, sanding and finishing. This weekend I had several filler-and-fixer needs. Knots are common enough and especially dead knots like the one below. Often the knot is fairly firm but some of the wood is pithy and punky. I usually leave as much of this rotted wood in place as possible but leaving it alone and putting on the finish will not resolve its lack of integrity and integration with the main body of wood simply because the sub-fibres of the rot are weak and mostly fractured. This I first resolve with a super-thin CA (super) glue which hardenes the compromised knot to make it knot-hard again.
Applying the liquid directly to the knotty area allows the fluid to pull itself into the fibrous area. A second application a minute after, will leave a heightened area above the surrounding wood.
Use a card scraper to scrape down any excess flush with the surrounding surface but don’t sand at this stage, it’s not necessary. I have always liked the idea of minimising sanding and relying more on card and cabinet scrapers which give as good if not a better surface when the scrapers are sharpened correctly and kept sharp.
I have always preferred wax filler sticks for a wide range of filling and fixing needs. You can intermix the colours for a match to the area you need and the collou remains the same long term. This is where I filled the void with a wax filler rather than more messy epoxy or transparent super glue which I often do too.
A last pass with the card scraper levels everything and you have a great fixup ready for applying the finish.
Search on somewhere like Amazon for wax furniture fillers. There are dozens to choose from. Some expensive and some in a pack of six light wood shades, another in medium shades and another in six dark wood shades. You can get packs of fifty different shades if you want them. The main thing to remember is that most woods darken over the years when exposed to light but then some woods lighten. The choice you make now will possibly stand out later but that is the same with any wood filler.