For a backer for sanding, there are a few things to use. WoodCraft and other wood working places sell a hard felt block or a cork block, which work well for backers. Or you can get those rubber sanding things that have a flat bottom and are curved on the top with the little spikes in a slit in the rubber on each side that hold sand paper inside the slits. I buy them and then cut them in thirds so I have the solid flat piece from the middle. Very firm and works well as a backer for sanding. I keep one long end square and I round the other long end.