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Rust prevention

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Here's a question I've never really found an answer to. Everyone says all rust must be removed, like cancer. However, there is no evidence to support this objectively that I've ever seen (strong opinions are not evidence :smile:)
Basically, iron will oxidize to various iron oxides in the presence of oxygen and basically catalyzed by the presence of water.
All coatings seem to work at rust prevention by sealing out oxygen and water, to a greater or lesser extent. This then prevents the rust.
Nothing in this equation relates to the presence of existing rust/ferrous oxides. Doesn't seem like this should make any difference except when the rust is thick enough to interfere with the sealing properties of the coating. I think this is why POR 15 (paint over rust) can be effective when painted directly over existing rust. It is designed to seep in and seal effectively in this situation. The point has nothing to do with POR so much as the point that existing rust doesn't catalyze the formation of more rust. It is simply oxygen and water from the environment that must be kept away from the iron/steel.
If this is true, then removing existing rust is only important so far as it is necessary to allow optimal sealing. Obviously loose, powdery, heavy pitted rust will prevent an effective seal. However, minimal rust at the bottom of deeper pits is unlikely to effect the seal of epoxy primer, it seems to me.
I don't "know" this, I have just thought about it a lot and done some quick and dirty "internet/google" research. There is plenty of opinion from experienced guys that all rust must be removed. However, no evidence or even theory given as to why that is. Generally just comparisons to "cancer" and the like, which is silly. That and a general feeling that it makes "sense", which we all probably share, but there are many examples of things that "seem" correct which actually are not.
Interested in opinions and thoughts on this.
Fred
 
OK
I just sprayed the phosphoric acid (in this case Eastwoods prep and prime, but I've also got some JASCO prep and prime from Lowes, seems like the same thing, and ACE hardware carries Ospho, again same same).
The limited residual rust at bottom of pits is easier to see as it turned dark brown.

Seems like the shiny steel is a little dull which I assume is what I want (etching for paint adhesion)?

No takers on the rust removal question above?

Fred

btw the JASCO bottle warned about contact with CONCRETE, so I quickly put some water down with a mop on the concrete floor that the prep and prime dripped onto.
 
A couple things Fred. I'm neither a scientist nor a chemist. My opinions are based on past experience experimenting with many, many different techniques.

First of all, the few times I've used POR15, I've not had satisfactory results and I'm not even talking about painting over rust, but on pristine metal. I had some suspension pieces I painted, then boxed and kept on a shelf inside my garage for a year. After one year, I found surface rust developing under the paint. I have a couple other products I have had much better results with and use instead.

Related to your question, is that I speculate that iron oxide that is trapped below the surface of the ferric oxide coating, , especially of crusty, still contains some moisture and oxygen and so will continue to be active to some degree.

Lastly, and for the same reason, I just prefer not to paint over rust, even when the conditioner is used, because I question the stability of the coating and hence the paint adhesion. I do a lot of paintwork myself and it is so time consuming and costly to do the finish work, that I just won't take shortcuts on the prep.

So, I guess I've not really answered your question, it's still just my opinion... YMMV, but I hate doing the same job twice.
 
Sandblast it and paint with two part epoxy. Problem solved for your life time.
 
The "complete" versus "mostly complete" rust removal question is mostly just for some fun discussion. Generally, I would like to remove all traces of rust before painting. Still not sure obsessing with every trace is necessary though.

I think Jack is right regarding "just blast and paint with two part epoxy".

I used the phophoric acid product all over my engine bay. The good news is it turned existing traces of rust black and hard (ferric phosphate), and made the shiny steel more like galvanized steel in look and texture.
The bad news is there is now some surface/flash rust that did not exist before, even though the car has been sitting in my shop in bare metal for a couple of weeks now. Two weeks bare but dry and no flash rust, 3 hours after phosphoric acid and now some appears. I think it is because the phosphoric acid is water based. The effect of wetting the metal with this solution has some acid-etching effects on the shiny steel, coats it in phosphate which retards rust a little, and converts the surface layer of exist rust to ferric phosphate. However, the water in the solution is a killer for flash rust.

Seems like the phosphoric acid may create more work for me when it comes time to wash it off and prime, in that there will be some surface rust to remove now.

Fred
 
Oh and thanks for your insight, Gerard.

40 years of experience trumps a bunch of theory any day. Food for thought is all.

Fred
 
Thanks Fred.

The flash rust is very superficial. All you need to do is take a 6 inch wire wheel in an electric drill or take some red Scotchbrite to it and it'll be gone in an instant. Re-wetting a wiping it dry after about 3 minutes will also do it. If you wipe it dry, you shouldn't get the flash rust back either. I never have that problem with the brand I use, but there are some brands that have a lower percentage of acid and hence more water, so that's entirely possible. I have seen it with some products I've tried.
 
I had a lot of pitting in my engine compartment. On one of the boards I found a mention of Bondo Glazing and Spot Putty. It goes on like spackle and sands just as easily.

I can't vouch for the longevity since I'm still working on the paint, but it allowed me to fill and smooth the pits with no special tools, and made a huge improvement in the final paint job.

It works fine over paint. I applied it over a primer for rusty metal where necessary, but mostly self-etching primer.

Michael
 
Michael,

Many of those you mention are lacquer based and not sure about stability with todays products. I can highly recommend Metalglaze by Evercoat, which is a catalyzed product. It's used by all the pros I know, and I have personally used it for over 15 years.
 
Thanks Gerard, as always.

I wound up using an acrylic enamel from CPC for the custom cherry red color. Sprayed it from cans and Preval sprayers only because the job grew from one or two panels to the whole engine compartment. Results were mixed (no pun intended).

I'll have to wait for the next 30 year freshening to try the Evercoat in the engine compartment, but the exterior still needs to be done... I will keep it in mind (and buy a proper spray gun).

Michael
 
Don't forget to lay on some clear coat before you put the engine room back together.
 
Jack-

The enamel has a "wet look" that seems out of place in the compartment. If I use a clear flat paint will that tone it down?

Thanks.

Michael
 
Tulsafred: You must be using a product that is mostly water. I use PurplePower's Deruster (about $6.00/quart at Advance Auto Parts). Here has been my experience: A little over 2 years ago I used it on 2 fenders that had been chemically stripped. After a little bit of experimentation I found that it worked best by applying, (I used a cheap paint brush and spread it on just like water), then waited about 10-15 minutes, then wiped off the excess. (If you leave the excess on, it will probably residue up with a white powder). The fenders sat for about 1 year (inside a covered building) subject to extreme temperature changes and high humidity. After that period of time, NO RUST! Not even surface rust.

After that one year mark,and out of curiosity, I sanded one of the fenders with 200 grit sandpaper. It has been over a year since then and STILL there is no rust (except for one small spot where it looks like someone may have touched it with their finger) and that fender still looks shiny new.

Apparently, this stuff micro penetrates the pores of the metal because that fender is still rust free even after sanding.

I'm tickled to death with Deruster and won't use anything else. Cheap, and easy to find, and works like a charm. Why pay more?
 
Michael, it should have a glossy shiny look. The factory painted em with the same paint as the exterior.

Like I said some clear coat before you start putting it back together will keep it looking like new for many years.
 
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