I have been making some projects that involve a ton of glue joints, and I am mating woods of vastly differing hardness. When I sand through all my grits (Festool RO 150). I get a smooth surface, but not an even surface. The more dense woods are proud. I show an example product below. The entire surface is a gradual curve in one direction or another, but I would like to have that completely flat surface as well as a silky smooth one. What should I do differently?