Treating rust on cast iron keel with rust convertor

C

Crinan12

Well-Known Member
Joined
6 Mar 2019
Messages
606
Visit site
Hi
When sanding rust back to bare metal on a cast iron keel, is there any point in applying a rust convertor after sanding but before painting/antifouling?

The rust has been removed so the rust convertor seems pointless - or am i misunderstanding the point of the rust convertor?

I read about rust convertor on a lot of posts in relation to rust on keels and I'm a bit confused about the purpose of it.

Thanks
 
the rust converter is usually phosphoric acid and is a good surface preparation anyway it shouldn't do any harm. might as well use it.
 
What I do every year is remove any flaky bits (usually zero) apply Fertan to any rust spots and apply a coat of metallic primer before anti foul. I’m having hull below waterline sand blasted this winter to remove flaking layers of anti foul but no intention of sand blasting the keel. By the time you get some epoxy on it it’s already rusting again so self defeating exercise in my view.
 
I beg to differ with eddystone. In my view, the best way to a relatively rust free keel is to grit blast it, then pressure wash with fresh water to get as much as practicable of the chlorides out of the pores, which will lead to gingering (light surface rusting) which can be removed with another quick grit blast. Next, prime with a good rust and moisture tolerant epoxy primer.

The first of these magic paints was Corrocoat, developed by a small Yorkshire company for use on N sea oil rigs. It dominated that market to the extent that 3M bought them to get their hands on the patent. The patent has now expired and many paint companies make lookalikes. 3M still make a further development, Scotch Kote<fancy number) and is probably the most expensive of the options.

We adopted this approach 11 years ago for our rusty iron keel, blasting the antifoul off the grp hull below the waterline at the same time and using the same epoxy primer on the grp where it serves as an osmosis barrier. 3 or 4 coats, I forget which. It has remained rust free since then and the Coppercoat finish has remained intact.

The fancy epoxy primers are not cheap, but 11 years of no spend on antifoul and no annual blue hair day make that a good investment, in my view.

Before we did the job, I telephoned Corrocoat, already acquired by 3M, and spoke with one of the team who developed the product. He said that my plan to pressure wash was great as anything to reduce chloride contamination would improve the paint adherence. When I asked whether a rust convertor (Fertan) would help, he was adamant that either residual phosphoric acid or phosphates would interfere with the curing of the primer, so best avoided.

Peter.
 
Agree with the above - although if you aren’t bothered about (a bit of rust) and don’t plan on epoxy coating, the converter and one component primer route seems to work for many on here.

Our keel was wet blasted, then we used the massive blasting compressor to dry it as best we could, before immediate zinc rich epoxy primer coating. Then many more coats. Epoxy fairing compound. Primer. AF.

Hoepfully it will last for 20+ years like the original epoxy coating.

I doubt it would last a third of that time on top of an off the shelf rust converter. The epoxy says to apply direct to abraded and contamination free metal - this is definitely not what a rust converter leaves you with.
 
I used rust converter when I cleaned my keel last winter. Used a Tercoo tool for initial rust removal then Fertan then primer and antifoul. There was small bits of rust showing when hauled out a month ago.
 
The problem with not worrying about a bit of rust is that when the rust grows, it blows off whatever finish is on top of it and allows salt water access to the slightly porous cast iron.

If your going for a long term antifoul option such as Coppercoat, that is not really acceptable.

Peter.
 
PetiteFleur said:
I used rust converter when I cleaned my keel last winter. Used a Tercoo tool for initial rust removal then Fertan then primer and antifoul. There was small bits of rust showing when hauled out a month ago.
Click to expand...
This is the kind of post that makes the Fertan thing intruiging to me, what does it really mean? There are many here that say "I treat spots with Fertan every year, works great", but to me the fact that there are spots every year suggests the opposite.
Rust isn't contagious, like some kind of disease to be killed off with bacteriocides. It's not the presence of rust that causes further rust, it's the presence of salt and moisture within that rust - yet it is the rust that gets treated. I just don't understand the logic of using the stuff. There's sod all that will neutralise salt, so copious rinsing, then drying, the sealing is surely the way to a lasting result - as a couple of posts above indeed suggest.
 
In the absence of blasting it's unlikely they're doing any harm, but whether they do any good or not would require you to do one side of the keel with and one without. :unsure:
 
You can test the prepared surface for chloride( actually any iron salts) with potassium ferricyanide papers.

.
 
Personally I wouldn't bother. Get to bare metal surrounding the area, clean with solvent, get some 2-pack metal primer on straight away. One or two more coats, couple of coats of regular epoxy primer, fill and fair as needed, vinyl primer, antifoul.

Did my whole (quite rusty) keel 3 years ago that way and only a couple of minor rust spots now with no further significant work.
 
When taking over my previous boat the vendor gave me an extended and thorough tutorial in all aspects of its maintenance; he recommended mechanically removing all rust spots, then allowing the bare metal to "ginger", as alluded to above, then apply rust converter, which would react with the developing rust to form a stable substrate over which to apply metal primer.
With my present boat I discovered that the best method of mechanically spot- removing rust is to use a Needle Blaster, such as can be hired from HSS. Having used one in 2019 I haven't had to deal with rust since, fingers crossed for January's lift-out!
 
Thanks for all the comments

Doesnt seem like their is a definite answer so I'll prob just take the easiest option i.e grind off, primicon (because i have some) and antifoul
 
The grit blasters fertanned my keels
Immediately after blasting and I had no rust even after a month. Washed off the loose stuff and epoxy faired the keels before 5 coats of gel protect and 4 of coppercoat. On the drying grid after 10 months and no rust anywhere. No fouling other than slime either.
 
Cast iron keels are slightly porus to water. This means any treatment can never completely stop rust. The OP was talking of sanding the keel. This will still leave some existing paint in the micro hollows in the casting. It would be highly unlikely for any preparation method to get a 100% clean metal. So then the choice is what to apply.

On Concerto I favour the Fertan route and over the years I have seen a declining number of rust spots. The keel was double treated with Fertan and washed off with water as instructed. The Fertan forms a coating to the cast iron that stops rusting so water does no harm. The next application is a minimum of 5 coats of International Primocon. This creates an impervious layer over the Fertan, finally 2 coats of hard antifouling (dark blue) followed by 2 coats of self polishing (white). Concerto normally remains in the water and is lifted every other year. Any small rust spots are cleaned and treated with Fertan and Primocon as above. One point I should mention is the bottom of the keel is impossible to treat as she stands on it. There is very little spread of rust coming from the bottom onto the sides of the keel, so the treatments applied seem to be very effective and well bonded.
 
You must log in or register to reply here.

Other threads that may be of interest

Sea Change
Mini sandblaster for rust spots on keel?
2 3 4
Replies
63
Views
1K
Mister E
M

Share this page

Top