If you do come across any felt tip drawings, a very good stain blocking paint is zinnser bullseye. Done a massive hall stairs and landing for a client a few years ago. Their annoying son would spend the evenings drawing things on the walls after we had finished for the day. A quick coat of bullseye and removal of the offender from the house for a few days let us at least get the job completed. I shudder to think what it looked like after he got back though.