squeak squeal squeak

For the life of me I can’t get the brakes on Mindful Mule to stop squealing. I’ve tried toeing in and adjusting in several different ways. I’ve tried rubbing the contact surface of the brake pad against concrete to roughen up its surface. I’ve tried riding down hills with the brakes slightly on to wear them in. Nothing seems to work. Luckily, they only squeak under hard braking so I can still get around town just fine and if I have to slam on the brakes then I get the added bonus of an ear-piercing squeal to warn of impending danger. The new rims seem to have a slight film on them that was there when I got them – maybe some part of the production process. Maybe I need to get that off of there. Dan the Man said something about cleaning the rims with toothpaste. I guess that makes some sense and probably wouldn’t hurt. Any other suggestions out there before I try new (softer) brake pads?