Haven't done this for some time but I'll bet you can still have rattle cans filled with the correct primer at the PPG store. It is costly but works better than a roller deal. Remember that primer is just that. A coat of paint that prepares a surface to hold the finish you intend as the final finish. For the most part primer will not completely protect metal or any other material. That is what the paint coat is meant to do. I have a friend that painted his frame with rattle cans filled with the correct primer and DuPont Imron. Looks great after about eight years. It can be done, if you really want to...