After years of making my own sanding blocks for odd jobs, I finally broke down and ordered these pre-made sponge core sanding blocks. And I'm glad I did. Although some use them for wet grinding, I prefer a solid core block for this. I prefer to use these sponge core blocks dry when I need to sand a surface quickly and I want my surface to be as smooth and flat as possible. a bit of sanding, but I want my surface to be relatively flat. These Maprial units are very cost effective for the budget conscious user.