Join Date: Sep 2012
Location: Wilkes-Barre, PA
That's oak ply and you sanded thru the veneer, I'm afraid. More sanding will only make it worse. For future reference, I never go lower than 80, and that's only for surface leveling when sanding solid wood glue ups and such.
Start at 120 and go up in grit. Makes no sense to go the other way. The ROS leaves little swirls, and each successive grit removes the swirls of the heavier grit before it and smooths the surface further.
Couple things you can do, depending on your skill set and adventurousness. Re-Veneer the window sill with spray contact cement and a piece of oak veneer. That's what I would do.
If you plan on staining the sill, you could disguise the burn thru by drawing in some grain with the appropriate color permanent marker and stain lightly using successive LIGHT coats, blending as best you can.
Whatever you do, don't try to sand it out ;-P