(quoted from post at 18:58:55 02/17/23)
(quoted from post at 19:04:57 02/16/23)
(quoted from post at 16:59:40 02/16/23)
(quoted from post at 13:55:16 02/06/23) I have used POR15 for that. It is a paint. Just take off the loose rust before applying it.
OR15 sells a zinc phosphate pretreatment too. Zn phosphate is a bit more protective than iron phosphate, but a little fussier and more expensive. Iron phosphate is good, and on a larger scale might be more practical. Either one will deal with the flash rust that you get after sandblasting.
As showcrop said, a phosphate coating will make the paint last longer. The SDS for the Sterosol Milkstone Remover at TSC says it contains 40-50% phosphoric acid at half the lowest price I saw for Ospho, which also contains 45%. POR 15 Metal Prep (Zn Phos) is $47/gallon, vs $19 for the milkstone remover. For my money, it's a toss up between Milkstone cleaner and POR. Depends on how comfortable you are with off label use.
When using the milkstone remover and you see the areas where there is just thin barely noticeable rust turn black, it is reassuring that you are getting the job done, and that you are doing the very best that you can do to keep it from rusting again.
Applying a shade tree mechanic coating is not exactly doing a proper job, but it is a lot better than nothing. An ideal coating gets complicated. As with any process, there are ways to screw it up and ways to improve it. You should dilute the phosphate solution for use on clean metal and obtaining the "best" coating, but you can get a reasonable coating if you don't. Just not something that you can sell to a customer who has a good QC department and needs documentation.
This is the simplest application description that a quick search coughed up.
AllPrep is a liquid, Iron Phosphate cleaner/conversion coatings
It is not one that I have used, but they don't over hype their product and they do adhere to the KISS principal more than the other sites I saw. I have no idea how this material stacks up to any other, but the web site is readable and reasonably informative. They start with a concentration similar to milk stone cleaner, etc., dilute it 20:1 and heat to 120F. If you have some rust to clean off, and don't want to operate at 120F, a more concentrated solution will do both clean and coat, but may not produce the best possible coating. meh
Iron phosphate solutions do work with spray or wipe on application. There are formulations with catalysts that may work faster and better, but a reasonably effective coating can be obtained with cleaning solutions that are not specifically formulated as phosphate coating solution. i.e. iron phos is very forgiving.
Zinc phosphate has to be applied with a proprietary solution. I only know the one readily available wipe on one from POR which I do use myself. If I ran out of the zinc phos and had a bottle of milk stone cleaner handy, I'd use it.