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Re: Ospho and epoxy primers

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Author  [Modern View]

10-09-2019 06:47:46

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I perceive your mention of acids as a must have, whereas this is not always the case. I talk of sand-blasting as it is a must have, but there are other more labor intensive ways to prep. To make this more clear and find common ground -- I believe sand blasting is the best, most effective overall prep work but may leave rust in places that can sometimes be best dealt with by properly using an acid product.

The original post was in regard to what type of primer to use over the acid product.

Let's study the TDS from Rust-Mort more closely:


A black coating indicates that there was a sufficient amount of Rust Mort applied to the panel to convert but not

remove the rust completely. This usually happens when the user stops the removal process before completion. This

black, insoluble coating is suitable for further top coats as long as there is no rust present underneath."

This piece of instructions can be misleading. How do you know there is "no rust present underneath"? Easy! -- you scuff the existing phosphate to reveal either bare metal or more rust. Hence, what this product is *really* telling you is that it is intended to be used to remove, not convert, but this paragraph may lead people to improper usage where they are actually hiding rust. Neither of us want that, and it is a "per the instructions" quote. Yes, Rust Mort converts the rust, but NO, it does not do it in a way that is actually leaving a phosphate suitable for topcoating and they allude to that in their own tricky wording. Rust Mort should be used to remove all traces of rust, completely. I think you agree with this based on how you have outlined your own use.


Studying the Ospho instructions will show you various different instructions, open to some interpretation (i.e. "heavy" rust -> Merriam Webster doesn't define this). Some of these instructions look really good to a rookie user as they promise a wipe on, conversion coating that can be topcoated. NO WAY will this leave you happy. Again, I think you agree with this based off of your last statement; Using it this way 1. leaves rust underneath the so-called phosphate coating or 2. leaves acid residue and wrecks adhesion. This is why Ospho also says to "test when using an epoxy primer" over their product. They are trying to cover their A$$ while still making things look super easy.

I am not against these products, I am against people being led to think they *have to* use them. I'm also against the misleading instructions of the labels. There are dozens of products out there that boil down to different %'s of phosphoric acid being peddled with different promises but in reality need to be used the same to dissolve rust away and leave bare steel.

I use the "prep and etch" one from Home Depot as it is about the cheapest and easily most readily available phosphoric product I can get my hands on. I've also used Naval Jelly (which HD also carries) but I don't like it and it is wildly overpriced for the amount you get. In reality, Ospho and Rust Mort are extremely overpriced as well. Sure, Rust Mort should have a higher acid concentration. Comparing 35-45% of the Kleanstrip product vs 40-60% of the SEM product doesn't show me a logical pricing of 10x.

In any case: Phosphoric acid = Rust REMOVAL. With any one of these given products, you need to follow generic phosphoric acid instructions that tell you to remove rust completely and rinse/neutralize the product while still wet, leaving a perfectly clean piece of bare steel. Following the label instructions on these products will leave you disappointed in certain cases and this is what your friend did with Ospho.


Things like paintron were talking about were a TRUE conversion coating that created a phosphate coating on bare steel (i.e. "parkerizing" solutions). These leave a true conversion coating not just "a layer of converted rust on top of other rust." The problems with these are:

1. price

2. availability

3. even more complex usage

4. lack of necessity

I think you agree on these points as well, since you aren't using these parkerizing products, you obviously don't think that we need a full on phosphate coated substrate on our tractors.


I don't doubt you had the experience with the pieces your hay guy did for you. I don't doubt that your new process has avoided this type of issue. My instinct and experience tells me that those pieces had numerous shortcomings in prep work. I do not want people to think that they need to prep every square inch of something with acid.

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