It wasn't until I noticed that my once tight-fitting dados were now loose that I realized that I almost sanded through the top layer of the plywood in my efforts to remove the marks.
You're not the first person to do something like this striving to erase layout marks or notes from wood by sanding and you won't be the last. It's the kind of thing that clearly indicates how poorly suited to the task the method is.
Now that I've tried other methods and seen how well they work I would never, under any circumstances, recommend removing pencil marks by sanding.
If it is viable for the removal of wood to be the means to remove layout marks from the surface (i.e. if any reduction in width or thickness is not critical at that stage in the building process) then both planing with a finely-set plane or card scraping would be preferable to sanding.
But it's necessary to have a method that can be used in the many instances when you can't safely pare some wood from the surface, and the best of those in my experience is by wiping with acetone; nothing else even approaches its effectiveness.
I don't know quite how it works but it's something to do with the specific solvent action of acetone and how it interacts with graphite. To use: dampen a corner of a clean rag or a bit of paper towel with the acetone, wipe and pencil marks just vanish.
I've never used acetone on marks left by lumber crayons but it should work on those too. However, I believe they are wax-based so it's likely that just mineral spirits (UK: white spirit) will be effective with those. Another possible method for those posted at bottom.
To be comprehensive I should include other methods that haven't been mentioned in the existing responses:
- Common pencil erasers; these can leave a dark smudge on the wood surface just like they can on paper so I can't recommend the method unreservedly, although they can work well.
- Putty erasers; these can leave a slight greasy trace on paper and consequently can on wood too so again I have reservations.
- White bread ball; sound bizarre but it works well to remove normal crayon marks from painted wall, wallpaper and other surfaces. Just rip off a chunk of white bread (avoid crust), form it into a tight ball and rub on the marks and away they go.