Biro on wooden doors - how on earth do I get it off?

[burstingbug]

DS1 has gone through the house with a biro or 2 just lately and has drawn over our wooden doors and on some walls.
Walls are easily sorted, I can paint them, but how on earth do I get biro off of the doors?
We rent our house so we have to be careful not to ruin the doors completely.
Any hints, tips or advice greatly recieved.

sandpaper

really? I was hoping to avoid sandpaper if I could.
DS1 is a real pain, no matter how hard I try to hide pens, he always finds a rouge one and heads start to the doors

I mean straight, not start

What sort of wood are the doors made of - is it varnished, or waxed, or painted in any way? Or just completely bare wood?

bare light wood, possibly pine?

Message withdrawn

You could try some of these (but test in an inconspicuous area first).