As long as I have been involved in woodwork, people have told me "don't worry about a small mistake, you can fix it with wood filler". But my experience with wood filler of any kind has given me poor results. I decided to try a few combinations of shop-made fillers and one version of commercial fillers, just to see how they work and how they compare. Note: Other Links as talked about in the video are below!!
Watch it on Youtube: https://youtu.be/KxnO7re7IOU
Remember too, there are other ways of fixing mistakes and defects in wood during the build process, wood fillers are only one way, and may not be the best way, but sometimes they are the only way ...
The first thing I needed to do was to prepare all the ingredients, which mostly consisted of very fine sawdust such as that which resides in the dust collector catching bag of my sander. I don't know how many times I have been told ... "Just collect some of the sawdust from your sander bag". So I did that, and it turns out that sawdust is useless because it is a combination of all sorts of woods you have sanded. What you REALLY need to do is to sand the wood you are going to want to fill, and collect THAT sawdust from the sander bag to use. That is the only way of getting something close to matching sawdust color for making a filler.
Below - Walnut Sawdust, Red Alder Sawdust, Mixed Sawdust from no distinct sources.
I decided to try 4 combinations of wood filler
1 - Mixing yellow wood glue with sawdust (Easily the most popular)
2 - Mixing Shellac with sawdust (I made fresh shellac just for this test)
3 - Mixing 2 part Epoxy with sawdust
4 - Using one of the commercial brands of filler
The next thing was to drill a series of shallow holes in a light-colored and dark-colored wood and to have the matching sawdust for each, which I had already collected.
I started off by mixing the shellac with the sawdust and mixed a crumbly kind of mixture and began filling the designated holes in both the dark and light woods.
Next, I mixed wood glue with the sawdust and this time the mixture was more of a putty-like consistency
My third mixture was 2 part epoxy mixed with both the sawdust and filled into the designated holes.
The fourth and last test was to use a commercial filler, in my case LePage Filler, and I had selected one of the lightest colors because it says on the package that it can be "stained", so I am assuming it will take a variety of colors of staining so that I could use this with different colored stains ... hopefully??
After filling all the holes in my test board, and waiting 24 hours for them to dry, what I found was this.
Most of the fillers had shrunk in the drying process which in some cases made them lower than the surrounding wood.
The shellac version did not shrink much and was very easy to level with the wood.
The wood glue version sanded level with the wood with a bit of effort.
The Epoxy version is very hard, sanding is NOT a good option, so I used a flat file and a coarse diamond stone to get it as close to level with the wood as I could without gouging the wood, then used my random orbital sander to level it off as best I could.
The LePage's was the easiest to sand level.
Next, I divided the boards into thirds so that I could use each section for Stain, Wood Dye, and Natural, this way I could see which of the fillers, if any, would absorb any coloration and to what degree.
The final step was to put a topcoat finish on the board, in my case I used OSMO, but I felt that most top finishes would give a similar effect to the fillers.
Conclusions
The shellac was VERY soft, and although it colored up alright, I would never recommend this combination, especially for something like a table or cabinet top.
The Wood Glue and the Epoxy were very similar so I am grouping them together, they both mixed OK, there is NO absorption from the wood dye or stain with either and they both appear nearly identical in color when dry; both adhere to the wood very well but the only way of getting a color match with these fillers would be to first make the filler, then match the dye or stain to the color of the finished filler. One of the problems with wood glue as you can see clearly in the pictures is where I smeared a bit of wood glue filler on the wood, then didn't sand deep enough, it blocked the dye and stain from penetrating the wood, I hoped this would show up in some places in the pictures and it did, so now you can see what that looks like.
One thing to keep in mind when shopping for brand name fillers, is some are made to fill small imperfections BEFORE finishing, others are made to fill imperfections AFTER finishing, and there is a huge difference between them.
The last filler I tried was the LePage Wood Filler, which is the only filler brand I could find near to where I live and based on my research, this brand appeared to be similar to others on the market for the same kind of filling, which if filling BEFORE final coloring and top coating. This company does offer the same filler in different tints which I did not attempt to try and color match. This filler has a nice, easy-to-use consistency and applies easily. The one thing that I noted is that every hole where this was used, the LePage filler cracked as it dried, which tells me the base of this product is probably primarily something like a Plaster-of-Paris or Gypsum of some kind, which is then tinted, and possibly some other ingredients added, and all of this information helps us to understand how and where this product is best suited. The product was easy to sand and level and, as they said, it DOES take wood stain to a degree, but did not take wood dye as well.
There are many, many other wood fillers, many of which are for AFTER finishing and are more suited to filling nail holes and small aberrations, and not something I would even attempt to try and test or compare.
In the end, I wasn't very happy with any of the wood fillers, but I understand they still have a place.
Keep in mind when making or using ANY wood filler, regardless of the finish or even no finish that is applied to the wood, that wood is going to darken with age. How much it darkens depends on how much tannin is in the wood, and how much light and air the wood is exposed to. With this in mind, it's important to use tones of wood filler that are darker than the existing wood. Most fillers will not darken with age which means you could end up with a project where the filler is a lighter color than the surrounding wood, and a project like "screams" of "filler was used in this project" and is something you will want to avoid. Hopefully, this article and video will help you in your selection of wood fillers and give you a start on the knowledge of how best to use them.
One More Thing
I know round holes are not representative of filling holes (usually) but I was more interested in color matching and how the formulas compare and absorb stains and finishes. As I have talked about in the past, there is another alternative I haven't mentioned and that is CA Glue. A somewhat new product on the market from Starbond is a "Brown" tinted CA glue which is a very good alternative in some situations as shown below, such as stabilizing knots and cracks and being brown it tends to blend better with wood. 
You can read more about this product on the woodworkweb Amazon Page or click the image
And another example of where you want to have a darker "fill" than a lighter one.
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