I have not filled anything in a surface that large. I have been filling my woodturning projects for years with epoxy colored and textured with a lot of different materials.
I turned vases, and cut a ring into the side or edges, and filled it with epoxy colored with copier toner. I also took the colored epoxy and mixed it with key filings to give a bit or sparkle here and there after sanding. I have also filled a decorative band around the top of a box. All with no problems.
A few years ago, I went to a furniture exposition in an artist's area of Texas and they had a lot of furniture that had huge fills in it that were epoxy and colorant. The color of the day was turquoise, with little pieces of iron pyrite and marble in it. These fills were in mesquite which is known for "wind shake" which breaks apart the parallel grain of the fibers leaving small to large (1" !!) cracks. These guys have been doing it for years and their display benches showed the wear of thousands of butts over the years that had sat on their benches. None of their benches showed any kind of problems.
Here's what I can share from my own experiences.
- You need to make your epoxy pour monolithic. Buy an epoxy with a long open time so you can mix and pour all at once. Mix more than you need. A little too much is just right
- I have found that the area needs to be fairly deep compared to its width. If I were going to fill something that is 1/4 inch wide, I would cut the dado 1/2" deep. Epoxy used as filler can be very brittle, and more mass is better. I have seen poured strips that are narrow and shallow come apart after setting up
- Make sure you overfill. The epoxy will shrink, it is just a matter of how much... If you are afraid of staining, use narrow blue painters tape to define your edges, then sand the whole mess off, tape and all. Don't try to remove the tape separately
- When filling a larger decorative strip on a bowl or vase, I always, always, turn a dovetail into the bottom of the dado. This could easily be done with your router. Even though some of my turnings have distorted after drying, the epoxy stays in place due to the dovetail
- Use a colored filler; black, brown, red, white, whatever. You will have bubbles in your fill, no matter what. If you have a dark material filler, you don't have to be worried about someone looking into your fill and being able to see bubbles. When you sand off the epoxy and the bubbles are apparent in the epoxy fill strip, sanding sealer will fill them and make them impossible to see
- To mitigate the bubbles and to make sure you get your material into the dado/groove, pour out your material, work it in with a small applicator, then put your vibrating sander on your project. Just about three minutes of your "vibrating tool" will work out many bubbles and make sure that your fills are all uniform. Since you have bit more epoxy left in your mixing tub (you did mix a little more than needed, right?) you can identify a low spot and fill immediately
As you can tell... I might have done a bit more of that than I will admit to...
Good luck! Hope that helps.
Robert