I would think, the best way to stop rust, would be to start with a VERY good coat of primer. If you make sure the surface is prepped right, and then coat the whole thing with a good coat of primer, followed by several good coats of paint, it should last quite a while...
Finnegan - same techniques as used in auto body repair. Sand to bare metal and remove ALL rust and smooth out pits. Sand to at least 2" beyond the rusted area. Smooth the area with a finer grit sandpaper. Remove all dust. Spray with a good quality primer. Let dry. (I use a 2nd coat of primer - most don't). Make sure primer is dry. Then spray with a good guality paint. Use as many coats of paint as you care to, making sure each coat is dry before adding another. For a good, quality job I usually apply 3 coats of paint. Then lacquer if you want for sealing paint and add shine. Wax for protection and to keep off snow.
There is a product that turns rust into a hard black substance that is sandable and paintable. I've used for a few thing and it seems to convert the rust and stop it from coming back. The only problem is I can't remember what it was called. The first stuff I used was in a squeeze bottle. It was a mess and hard to apply. The last stuff I got was in a aerosol can. That is what I would recomend. It was easy to apply.
Hmmmmm, saw a 55 gal. drum in the chemical room at work the other day. It said "Acidene" on it. "Rust and scale remover". Now I'll have to read the label, and see just what kind of acid is in it. It might be muratic acid, but in my experience, muratic is used to etch concrete before applying paint or sealer.
One of the products that turns rust into a black sandable primer is called Extend. You can find it at Auto-zone, Pep Boys, Wall-mart .I've used it quite a bit over the last several years. It stops the rust from continuing the oxidation process (must be top coated, to last). I'd go with Mick's suggestion just use this before priming and prime with only one coat.
Turns out the packaging slip was still taped to the 55 gal drum of "Acidene". Along with the MSDS.
There are 2 ingredients listed. One is water, 82%...
The other 18% is HYDROCHLORIC ACID.
The instructions say to dip the parts in the solution, and promptly rinse with lots of water. It says it can be brushed on too, then rinsed. A respirator, rubber gloves, and a face shield are MUSTS. I should have know it was potent stuff on metal considering it's in a plastic 55 gal. drum.
~Chuck aka "toxic paint guy" and now "acid guy" too I'm sure.. ;-)
If it wasn't for the surface roughness, I'd get my plows galvanized. Pricey, but permanent. There's also the idea of having a silver blade to reflect light back into your eyes that would be a pain for the first year. After that it would be dulled down to a nice satin gray. Used to be a place in Everett, MA that did galvanizing in colors. That might be the way to go,, just might be worth looking them up and dealing with them, they used to have pretty much of a butthead attitude. Seems like that would take care of rust on the A-frame and semi-circle on that Western I'm bringing back from the dead.
I have used a paint on antique car frames that is brushed on. It comes from a dealer in Newark NJ called Bill Hirsh. It works better than extend and POR 15. It needs to be top coated, but it works well and lasts forever.
I like Alan's idea of galvanizing your plow. As a welder by trade, we build a lot of things that end up getting galvanized. Some years back we built a roof rack for one of our trucks out of 1-1/4" square HSS (Hollow Structural Section - tube) and galvanized it. No more rust problem. A couple of things to keep in mind if you send something in for galvanizing: The surface must be clean, so sandblasting all the old paint and rust off would be required. The other major consideration is venting. Galvanizing essentially involves dipping the part in molten zinc. Any sort of closed in area (tubular sections used on some plows come to mind) or corner (where the ribs meet the top angle on the moldboard for example) must have a hole or holes to allow the heated air to escape when the part goes into the zinc and to permit the zinc to drain out when the piece is lifted out of the tank. Without this venting, the result will be a rough, incomplete galvanizing job and there is also the risk of the superheated air (steam if any moisture is present) causing damage such as blowing out seams in the tubing. The shop that does galvanizing work will know what is required in the way of venting - speak to them for specifics! (Every job is different) You can paint the finished product too if you like, but you will have to do some extra surface prep to assure paint adhesion. I admit that with regard to the cost of having a plow galvanized I am "in the dark" - perhaps someone out there would have an idea? Still, this galvanizing idea has me thinking...