This is when you use the 40 grit flap disc on an angle grinder. I bought my flap discs on Amazon for $16 for a 10-pack. Ended up using three 10-packs on my boat restoration.
This is what my transom looked like after removing the wood and hitting the surface with a belt sander (36 grit) to get rid of as much of the remaining wood chunks as I could. Grinding with the flap disc takes it down even further to good glass and removes that paper thin layer of wood - like
this.
Prepare yourself for the ensuing dust storm. You'll need a respirator and eye protection at a minimum and I recommend long sleeve clothes and a Tyvek suit.