Phosphoric Acid ????

Well, there are many different phosphoric acid based products that seem to promise a lot. There are also many more opinions on how to properly use 'em. I'll give you my own take on the issue.

Don't expect too much from a treatment with phosphoric acid. If you have significant rusting to deal with there really is no quick fix for the problem and sandblasting to a bright metal condition is the most appropriate way to proceed. A next best way (IMO) with significant rust present is to use a treatment product such as Permatex (Loctite) "Extend" which reacts with sound rust to provide a hard polymer encapsulating barrier against moisture and prevents further rusting from taking place. I have used such a product successfully in a place where I did not want to sandblast.

The phosphoric acid treatments (liquid) available from the big paint manufacturer's such as PPG and DuPont are really two-step products if you strictly follow the manufacturer's instructions. PPG has DX579, Metal Cleaner and DX520, Metal Conditioner. Both have a phosphoric acid component to them and should be used together. The first product is mixed with water. The second product is used straight from the bottle. Here's the .pdf data sheet for these PPG products. Note that PPG does not recommend using on cast iron. DuPont has basically identical products, 5717S Metal Conditioner (cleaner) and 5718S Conversion Coating (conditioner). Both of these phosphoric acid based systems require a rather messy application: Apply the first product, scrub with an abrasive pad, rinse throughly with water, dry, apply second product, scrub with an abrasive pad, rinse throughly with water, dry and finally prime. I have used the DuPont products successfully in the past on light surface rusting. They work well for such light duty stuff but I would not bother if the rusting is on the heavy side (see above). I also would not bother using any of these products for freshly sandblasted (to bright condition) metal. I read somewhere (can't recall where) that use of phosphoric acid treatment on sandblasted metal was not recommended. I have done so in the past but have stopped the practice altogether in the last few years. I understand that the first product reacts with rust to form iron phosphate and the second product converts the iron phosphate to zinc phosphate. I am not a chemist so can't verify the actual chemistry here. As Rusty indicates though, I think it would be appropriate to use on sandblasted metal that did not get primed right away and had some flash rust develop. You should try and avoid this if possible.

In lieu of separate phosphoric acid treatment, many use an etching primer which has a phoshoric acid component in it. Many swear by this product but I admit to not being a fan of it, prefering instead to go with direct epoxy as a bare metal primer. I have always had the feeling that DuPont pushes the etching primer and PPG pushes the epoxy, even though both companies market similar products.

It's a somewhat confusing situation I must admit but the above are my thoughts on it.

Rod

When doing old sheet metal from the ground up, you invariably run across the need to use some plastic filler, even if you do a decent job of banging out all dents, etc. There are some experts that can work metal so smooth that you don't need filler but I certainly am not one of them. I also do not have the patience to even try! My work flow in this instance is to sandblast, prime with epoxy, apply filler where needed and sand. At this point I always have some amount of bare metal showing, having unavoidably sanded off some of the epoxy. Take a look at the PPG OMNI tech sheet for MP176, their etch primer. Notice that the only compatible substrate is "properly cleaned and sanded steel, galvanized, aluminum". My part now has three different substrates to go over; 1) epoxy, 2) bare plastic filler and 3) bare steel. How do I treat the now bare steel portions? The same situation would exist if I had used etch primer instead of epoxy. I would be left with applying more etch primer over a substrate (in this case, plastic filler) that is not recommended by the manufacturer. It would be next to impossible to spot just the bare steel, certainly not practical.

That problem does not exist when using epoxy primer. Take a look at the PPG OMNI tech sheet for MP170, their epoxy primer. Notice the compatible substrates: "cleaned and sanded steel, galvanized, aluminum, fiberglass, old finishes, body filler" plus a host of other PPG OMNI primer products. I can now put more epoxy over my example above, keep within the manufacturer's recommendations and avoid a possible problem later on. I suppose I could have used the etch primer first in the above example and after sanding the filler, use epoxy but what have I gained in doing so? I am still left with epoxy on some bare steel where the etch has been sanded off.

Some will argue that etch is the only way to fly and I agree that it certainly is a lot easier and faster than dealing with the messy two-step process with phosphoric acid that I noted previously, if you are going to "etch" at all. In any event different etch primers may have quite different rules for use so I think anyone using them should carefully review the tech sheet for the particular product to be sure of material compatibility. Use of etch on cast iron is somewhat questionable. It is not mentioned in the tech sheet (at least for the MP176) but I am not surprised at that since cast iron is not normally a part of autobody work, the prime market for products such as these. PPG does not recommend phosphoric acid treatment for cast iron. Since etch primer does have a phosphoric acid component, that might be a caution sign. My gut feeling is that I just wouldn't do it...but of course I am biased in favor of epoxy :o).

I have no idea of the cost difference. I used a DuPont etch primer once (Variprime, I think) a long time ago. I changed to epoxy shortly thereafter and never looked back. My guess is that the epoxy is more expensive but I have no real basis for that. Most likely you would have to put something on top of the etch anyway prior to topcoating...check the tech sheet for the topcoat you are using to verify that the etch primer is a recommended substrate. For PPG OMNI MAE acrylic enamel and MTK acrylic urethane, it is not.

As far as etch primer over sandblasted steel (bright metal blast), I don't particularly see the need or advantage over epoxy. Didn't someone...Cliff maybe...caution about etch over a blast profile recently? Something about it possibly becoming too thick? The MP176 indicates that a maximum film build of less than half a mil is to be used. That's not much considering that an epoxy or even a topcoat is about one mil per coat. My tendency would be to avoid it here also, but I am open to any thoughts to the contrary.

I have not seen much in writing about prep of cast iron for paint, probably because it is not a thing to be routinely encountered in the auto body shop world. The castings I have done to date have been either sandblasted or hot tanked as noted. Then epoxy and topcoat. My B though is going to have to take a route similar to yours.