If a pulley is squealing can you fix it by using sandpaper to roughen the wheel or should you just get a new belt?

As your belts are made of rubber, any kind of oil on it WILL deteriorate the rubber and eventually cause the belt to break. I do NOT recommend putting brake fluid on the pully. I would replace the belt as in the long run it usually ends up being cheaper than trying to fix the problem yourself. If the pulleys still squeal, then I would make a stop by a mechanic and have him check on the squealing pulley as there may be a more serious problem than you think. A friend of mine just ignored the squealing and kept driving the car, and a couple of weeks later the water pump shaft snapped and blew the water pump literally to pieces and sent rad fluid rushing out all over the road.... To make a long story short, he could have avoided the $450.00 repair bill if he had it checked out by a mechanic.

DO NOT POUR ANY OIL ON A BELT go to any auto store and buy a stick of belt dressing if that doesn't fix it, buy a new belt and for sure, do not take sand paper to it

I also agree don't even think about putting Brake fluid on it. I would check the tension on all the belts first before going out and buying a new belt. Over time the belts can loosen. They can easily be tightened . go to any parts store and buy a car manual. They have all the specs and give every detail on how to tighten and how much tension each belt should have. Also if you do use belt dressing it might stop the squealing for a little while , but once it dries up the squeal will return if in fact it is loose.