I am fixing a leaky trap under our kitchen sink and have a question about drain pipe fittings. I see several types of washers being used: hard plastic cone shaped ones, and soft rubber ones. Is there a rule for which type to use where? There seem to be two types of pipe connections: most are slip joints, but a couple are not -- like the one where the J shaped pipe coming from the wall meets the U-shaped trap. There is a rubber washer on that one, which has disintegrated. Should I replace it with the same kind?