my other half's darling daughter has covered all of her walls in grafitti - marker pens, pencils, makeup etc.  most of it has covered ok, but therer are 2 bits that just keep coming thru.  tried oil based undercoat, stain block, red oxide primer, grey celly primer.  anything else i can try to stop it coming thru, short of sanding back thru the plaster until it is physically removed?