Little thing about tar i found today, if your rims arent ground, find a hill, tar up your rims then go down it with the pad almost locking (you still need to go at a decent pace) so slightly clamping the rim, after a bit it will stop squeeking, stop with your other brake! and then look at the rim the whole thing gets a big black line all round it and the tar is super evenly spread, you need to be carefull you don't lock your brake or you will be seriously fooked!