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Old 25-11-2022, 11:52   #1
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Rust reformer on cast iron - will epoxy stick?

I'm in the middle of dealing with paint failure on my cast iron keel. I'm in Panama, and they don't have sand blasting in this yard, so I'm looking for a "good enough" approach until I can do it right in a more fully equipped yard with proper sand blasting.

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https://www.cruisersforum.com/forums...ed-269819.html




The approach I went with was to grind down to bare metal where ever there was signs of rust or failing paint. There is some texture to the casting, and some crevices where grinding and wire brushes couldn't get every trace of rust, so I decided to treat with rust converter. The only brand I could find locally here in Panama was Sur Corrostop.


The plan from here is to use a 2 part epoxy primer (5 coats), then underwater primer over that and all the areas of the keel that I didn't grind back, then bottom paint.


The problem is the rust converter. It has left a shiny residue that I'm suspicious of. I'm worried that it will act like a release agent and prevent the epoxy primer from bonding properly to the metal. I've included photos of some drip marks left from the rust converter that illustrate the sort of residue it leaves behind.



What do you think? Am I better off leaving the rust converter in place, or going through some sort of solvent/mechanical process to get it off? Remember this whole thing is supposed to be a stop gap solution for 1-2 seasons until I can do a total redo of the keel with sand blasting equipment.
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Old 25-11-2022, 12:00   #2
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Re: Rust reformer on cast iron - will epoxy stick?

Usually after apply the rust converter and let it dry, you must wash the residue left behind with water and as soon as it is dry apply the primer. Btw Interprotect work fantastic on those rusty keels..
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Old 25-11-2022, 16:14   #3
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Re: Rust reformer on cast iron - will epoxy stick?

Thanks for your input. The shiny residue left behind doesn't actually wipe away with a wet rag. It seems almost like a clear coat of some kind.
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Old 25-11-2022, 16:36   #4
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Re: Rust reformer on cast iron - will epoxy stick?

A couple of points to note -

Check to see what the primary active ingredient is in the Sur Corrostop. Presumably it will phosphoric acid plus a few other minor ingredients. If so, then the following applies.

Phosphoric acid will slightly etch clean steel (iron) (Fe)
Phosphoric acid will react rust (iron oxide) to form Iron(III) phosphate (ferric phosphate). This is the black surface you see after using most 'rust removers' or 'rust convertors'. Iron(III) phosphate blinds strongly to the underlying steel and is stable i.e. prevents further rusting of the underlying steel but not of any adjacent 'clean' steel.

Cast Iron is very porous.

To effectivity treat rusty cast iron you should

* mechanically remove as much rust as possible
* apply rust treatment (phosphoric acid) and do not let it dry.
* thoroughly wash off the remaining phosphoric acid. A high pressure water blaster is best for this.
* thoroughly dry the cast iron. Dry heat is best for this. If you have access to a gas torch (oxy acetylene etc), warm the keel in sections.
* apply the epoxy as soon as the cast iron is thoroughly dry. It is best to apply the epoxy to a warmed surface.

Because cast iron is so porous, applied heat will ensure moisture is removed from the porous surface and if the epoxy is then immediately applied to the warmed surface, the epoxy will be thinned (by the remaining heat). As the cast iron cools, the epoxy will be drawn into the porous surface.

Low relative humidity helps but that may be difficult to find in Panama
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Old 26-11-2022, 07:37   #5
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Re: Rust reformer on cast iron - will epoxy stick?

Great stuff, thank you! I donít think Iíll be able to do all of that right now right here, for example Iím not removing the paint from the entire kill making the torch approach problematic.Iíve also already applied the rust reformer, so no opportunity to wash it off before it dries. However I will definitely keep your advice in mind when I redo the keel comprehensively with proper equipment next time.
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Old 26-11-2022, 08:01   #6
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Re: Rust reformer on cast iron - will epoxy stick?

I have always heard that it is fine to leave the black finish, at least for non-keel uses. Indeed the the instructions say so. I've also had tolerable success with Rust-Oleum rusty metal primer for temporary touch ups.
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Old 26-11-2022, 08:35   #7
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Re: Rust reformer on cast iron - will epoxy stick?

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Originally Posted by JBP View Post
I have always heard that it is fine to leave the black finish, at least for non-keel uses. Indeed the the instructions say so. I've also had tolerable success with Rust-Oleum rusty metal primer for temporary touch ups.
Same here, the black finish once dried is a tough finish, so have just painted over the top with lots of coats of International Primocon which dries quickly.

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Old 26-11-2022, 15:31   #8
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Re: Rust reformer on cast iron - will epoxy stick?

Yep, that black stuff is ferric phosphate (FePO4) and is bonded to the underlying iron (Fe). It is hard, stable, but slightly porous so it requires further barrier coats (paint / epoxy) in aggressive environments (i.e. marine use). Do not overcoat with a zinc rich primer though, as FePO4 is a dielectric (electrically insulating). Zinc rich paints require an electrical bond to the underlying iron to work properly and the FePO4 prevents this.
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Old 26-11-2022, 15:47   #9
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Re: Rust reformer on cast iron - will epoxy stick?

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Originally Posted by ohthetrees View Post
Great stuff, thank you! I donít think Iíll be able to do all of that right now right here, for example Iím not removing the paint from the entire kill making the torch approach problematic.Iíve also already applied the rust reformer, so no opportunity to wash it off before it dries. However I will definitely keep your advice in mind when I redo the keel comprehensively with proper equipment next time.
If you can heat the treated areas by just a few degrees above ambient (hot air gun / hair dryer / etc) and hold it for say 10 or 15 minutes before applying the epoxy, you will get some improvement in performance or if it is a hot sunny day, wait until the max temperature of the day has been reached before applying the epoxy. The idea is to get the porous cast iron to be as dry (and warm) as possible and apply the epoxy as the surface is about to cool.

The problem with allowing the rust treatment to dry is that it gets drawn into the the cast iron as it dries and then it has to be power washed out of the porous cast iron or you end up trapping minute amounts of phosphoric acid under the top coating. In a perverse way, the rust treatment only works properly on a slightly rusted surface and it is disadvantageous to a clean rust free iron surface.
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Old 26-11-2022, 20:03   #10
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Re: Rust reformer on cast iron - will epoxy stick?

I find the best approach is to brush, grind, sand or chip the rusted area to get the scale off then apply a couple of coats of phosphoric acid then use a power wire brush to clean the area after it has dried and prime immediately. For small areas this is almost as good as blasting.
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Old 27-11-2022, 11:06   #11
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Re: Rust reformer on cast iron - will epoxy stick?

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Originally Posted by Wotname View Post
If you can heat the treated areas by just a few degrees above ambient (hot air gun / hair dryer / etc) and hold it for say 10 or 15 minutes before applying the epoxy, you will get some improvement in performance or if it is a hot sunny day, wait until the max temperature of the day has been reached before applying the epoxy. The idea is to get the porous cast iron to be as dry (and warm) as possible and apply the epoxy as the surface is about to cool.

The problem with allowing the rust treatment to dry is that it gets drawn into the the cast iron as it dries and then it has to be power washed out of the porous cast iron or you end up trapping minute amounts of phosphoric acid under the top coating. In a perverse way, the rust treatment only works properly on a slightly rusted surface and it is disadvantageous to a clean rust free iron surface.
What is the big downside to a little phosphoric acid being left over? Won't the tiny amounts of it left unreacted soon react with the huge mass of iron it is embedded in?
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Old 27-11-2022, 15:14   #12
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Re: Rust reformer on cast iron - will epoxy stick?

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What is the big downside to a little phosphoric acid being left over? Won't the tiny amounts of it left unreacted soon react with the huge mass of iron it is embedded in?
It is more of a minor downside. You are correct in so far the tiny amount of the acid left will soon exhaust itself by reacting with the cast iron however wherever this happens, the epoxy is attached to the surface of same cast iron. In essence, the acid is chipping away underneath the substrate the epoxy is trying to protect from external elements. Instead of the epoxy sticking to 100% of the cast iron surface, it is now only sticking to 99.999% of the cast iron surface. Sooner or later, some water gets to the 0.001% and that bit rusts. Rust expands (by about 10 times) and this loosens the epoxy coating on this tiny spot and the problem gets a bit bigger. This compounds the problem until you see the top coat has separated from the keel.

To reiterate - phosphoric acid on rust is a marriage made in heaven; phosphoric acid on clean cast iron is divorce material. As we can't keep the acid away from the clean cast iron areas, we have to our best to ensure the acid is fully washed off before applying the epoxy. Letting it dry makes it harder to fully remove it.

Of course the detail is way more complex than the simple explanation above but the long and short of the problem is the acid is easier to wash off when still wet.

From https://www.sciencedirect.com/topics...hosphoric-acid

3.02.5.4 Phosphoric Acid
Phosphoric acid attacks unalloyed cast irons at all concentrations. Austenitic cast irons are satisfactory when used at ambient temperature in solutions having concentrations less than or equal to 30%; above this threshold, the attack is rapid. High-chromium cast irons are suitable in most cases. The corrosion rates are ∼0.12 mm year−1 up to a concentration of 60%, at all temperatures up to boiling.69 High-silicon cast irons best resist this acid, at all concentrations and almost all temperatures (see Figure 28). Note that most crude phosphoric acid contains appreciable amounts of fluoride which can lead to excessive corrosion rates.
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Old 27-11-2022, 17:58   #13
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Re: Rust reformer on cast iron - will epoxy stick?

^^ @wotname, thanks for this wonderful discourse. I use rust converter on some iron keels, and all this is really valuable information.
Cheers!
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Old 27-11-2022, 18:08   #14
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Re: Rust reformer on cast iron - will epoxy stick?

Timely as I just applied some Ospho this AM to MILD STEEL.

Directions say to paint over directly. No mention of rinsing.

Was researching this topic a couple of hours ago in prep of painting tomorrow.

My past experience is that 80% of the time what ever I do works.

Yet there is always some areas of failure, no matter what I do.

Rather frustrating.

Stull hetter than being at a desk, or TV.
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Old 27-11-2022, 18:38   #15
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Re: Rust reformer on cast iron - will epoxy stick?

^^ I'd stick to the directions given by Ospho. Different rust convertors have different additional salts in the mix which does change the chemistry and how they react with different metals.

Mild steel is lot different (chemically) to cast iron and nowhere near as porous.
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