Fonawah,
I use a medium, red, non-permanent marker, no problem. You only need one dot, after all! Lick your finger and rub it off when done.
If you use permanent marker, a good solvent is acetone (the active constituent of nail polish remover, but without the cr*p, or price). I get it in Boots or Superdrug. Never tried meths (except on Friday nights, but that's another story
)
Using a pencil eraser is supposed to make the surface opaque over time, I believe. Not proved this myself.
Cheers
SD