Filling holes in a timber workbench top

Question

I got an old workbench with a top made by a pair of railway sleepers (I assume this is timber). It was very old and sticky with paint stains and not very plane at all, so I tried to use a router and a planning jig to make it nicer to use.

I've almost completed the process and it only needs a last router pass to be "almost as new" but it has a few big holes (about 10cm/4 inches long, see picture below) that I would like to try to fill to avoid them being always filled with sawdust or trapping screws and other small things that like disappearing in holes. I'm also concerned that since they are close to the edges the outer part may break away and make an even bigger hole.

I heard about something called "liquid wood", but also of "epoxy glaze" and some peoples seems to use wood glue mixed with sawdust, but I have no idea which of those solutions (or another one) would be the best in this use case.

I don't want to have a "mirror finish" (this is a workbench, and I'm planning to use it) but I'd appreciate if it could still look good.

My questions are:

• Which product can I use to fill those holes
• Should I fill the holes before the last router pass or should I finish with the router and fill the holes later (then presumably sand whatever I put in the holes)
• Extra question: My router has variable speed (6000 to 26000 RPM); what would be the recommended speed to use in this case for a better looking finish (I'm using a 1 inch diameter router bit like this one and I'm removing about 3-5 mm at a time)

I should add I'm pretty new with routers or woodwork in general so feel free to add anything that seems obvious to you.

Answer 1

I would use a wood patch called a dutchman. If your router has a plunge feature, make a small template to aid in cutting in a dutchman. The article the link refers to does it differently than I do, since it cuts out the damage and the repair piece with the router using one template and two different guides. Where I use one guide only to cut the bad spot out, and a table saw or miter box, or sometimes both to cut the repair piece itself, depending on how big the repair is.

You would be able to get a dutchman in plane pretty easy with the surrounding material, but as with any repair of this nature, I would do it before the last pass of the router over the whole surface, then sand everything uniformly afterwards for the fine finish. This will clean up any glue mess from the install of the dutchman.

The router speed to use on that is not critical, but for the larger bits, you can go too fast. Sanding will take out anything the router leaves behind. I would keep it a the lower half of your routers speed range 10,000 RPM max. The rule of thumb is, the larger the diameter of bit, the slower the speed. Some router bit manufacturers list the maximum speed a bit should turn. Other than that, listen to your router while it is cutting, its distinctive whine will change as it is pushed into the wood. Do not let it change in pitch too much. The amount you are cutting in one pass is a good amount, take no more on a full cut pass. As in each pass of the router cuts the full 1" width.

Learning to make templates and using bushing guides are one of the more handy things you would do well by.

Answer 2

If you don't mind not having the patch resemble the surrounding wood species; for a quick long lasting wood filler I've always used automotive body filler (Bondo). It is a two part product and once the hardener is mixed with the filler you have finite time to use it (depending on the amount of hardener used). Once cured it can be shaped with basic wood tools. The real benefit is it won't move (expand or contract), it isn't affected by climate (U.V. and rain-proof), and it's very inexpensive. The only possible down-side is it's color (pink to reddish) which can not be stained. It accepts any paint type readily. Another alternative that is extremely durable is to use any brand of 2 part epoxy glue. Since your wood surface will be subject to physical abuse, epoxy applied to any size holes can not be jarred loose. Also, it is liquid enough to be poured into the void to find it's own level and it will prevent any splits or cracks from further opening because of its strong mechanical bond. I believe you can find several finish/color types ranging from a clear 5 minute epoxy to the traditional grey color (I know there is a white color, too).