Thread: products I like

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Originally Posted by randyr

Uncle Bob, I'd like to hear more about this phosphoric acid rust treatment, whether here or somewhere else if that's too much of a hijack for this thread. I have a few rust bubbles near the rain gutter on my '66 F100 that definitely need some attention. I'm hoping they won't require major surgery but in the meantime, an effective treatment would be helpful. Thanks!! Great thread, guys!

I have bought straight phosphoric acid from a chemical supply house in a gallon jug (actually a 75% solution), but it's available in a variety of smaller packaging, in lower strength from auto part stores, hardware stores, and paint suppliers. Navel Jelly is probably one of the better known retail brands. From automotive paint suppliers a common brand is Rust Mort. Most "rust retarders" available have some form of phosphoric acid as the primary ingredient along with dyes, thickeners, whatever else the seller feels will add difference/magic to their product.

The way I learned it is to apply to a rusty or non=rusty area very wet. If there is rust present scrape/wire brush off most of the scaly rust first. I will use a cheap brush to apply the acid, avoid "plastic" brushes, they dissolve quickly. Some will use a sprayer. I prefer the control of the brush. Depending on severity of the rust I'll let it sit for an hour or a few hours, just make sure it stays wet. Sometimes if the rust is stubborn (say in pits) I'll use a wetted wire brush or Scotchbrite pad (these get eaten up fairly quickly though). If you let it dry you'll have a hard, plastic like coating that is a mess from my point of view. If I brain fart and let that happen I sand that mess off and re-do. Once I've decided it's done its conversion thing (typically a dark black film look with some white foam or crystals about) to iron phosphate I'll give it a good washing with plenty of warm water to neutralize. That's the step that freaks out some people; "Won't that cause it to rust right away?" No, not if you've allowed sufficient time for the acid to do it's job. Below are some splash shields from my old Mopar that had just a slight amount of surface rust in a few spots. I blasted all of the old paint off and treated the entire piece(s). All these were done at the same time in the same way, the color differences I attribute to differences in alloy makeup of the steels used. Done this way it is an effective plating process (watch films of platers, they use various acids for a similar outcome).

I have done this sort of treatment on bare frames during construction where I don't want to have to grind away paint to weld on brackets or such. I've had a frame sit in the shop for a couple years (yeah, I'm slow) with no visible re-rusting. As long as the surface was cleaned of the acid residue thoroughly, and then re-cleaned as normal before spraying, paint sticks fine, probably better due to micro etching of the metal surface by the acid (which is one of the points of using Rust Mort as an example).

EDIT: just in case, it should be said, this is ACID, be sure to wear eye, lung, skin, and clothing protection during use.