The technique is very similar to the sort of moldless composite work Burt Rutan pioneered in aircraft use. The fiber-reinforced plastic is formed around a positive ‘buck’ that remains inside the final part, adding stiffness and some backing. Unlike Rutan’s designs, which used pure Styrofoam inside, [Whitburn] is using a sandwhich of two layers of thin ply (3.6 mm) with XPS foam in between. That’s also where you have to be really careful to avoid oopsies– go through the cotton and you have to start over. That’s pretty good all things considered– a lot lighter than a cement countertop, or even one made of DIY resin tiles.