I just moved into a flat with walls that look like they have decades worth of tobacco smoke stains. I'm on a limited budget and I wonder if an oil-based undercoat as opposed to expensive primers out there would help to cover this mess? Forgive me if what I asked is a stupid question but I'm an amateur and I've only ever painted walls that have been in good condition. I've used oil-based undercoat on wood in the past and it worked well which is why I am wondering now if it would work on stained walls. Thanks if you can help.