Treating patches of rust on cast iron keel

[Kelpie]

We're currently on the hard in Spain and I've noticed a few places where the existing coating is peeling on our cast iron keel.
I don't think it's anywhere near bad enough to do a full strip down, and anyway we don't really have time for that sort of thing.
Assuming the proper course of action is to grind back to remove anything loose and expose bright metal, then treat with suitable rust treatment, followed by epoxy barrier, then AF.

1- is a grinder better than flap disc or wire brush?
2- what's a suitable rust treatment? I'd probably be buying from SVB24 and the choices are a bit confusing. SVB | Online shop for yacht and boat equipment
3- should I apply a layer of zinc rich primer to the bare metal?
4- is normal epoxy resin ok (have some on board) or should I be looking at a specific epoxy primer paint?

Thanks!

 

[MoodySabre]

I use a flap disc on a grinder, rust converter, undewater primer then AF. Lasts a couple years at least. I just use Vactan which is like kurust but a lot cheaper. Fertan is a different thing and probably better.

 

[Tranona]

Don't bother with epoxy, unlikely to get it clean enough for it to stick without a complete blast. for smaller areas a grinder is probably best, although others favour flap wheels. once clean conventional primer such as Primocon and antifoul. If there are pits that annoy you then you can use an epoxy filler after the Primocon. With iron keels unless you blast I think you just have to accept that anything you do is just regular maintenance rather than a permanent fix.

 

[vyv_cox]

Don't use a wire brush on cast iron. It tends to drive rust into porosity and graphite. Grinder is my preference but flap disc not too bad.

 

[Spirit (of Glenans)]

Don't use a wire brush on cast iron. It tends to drive rust into porosity and graphite. Grinder is my preference but flap disc not too bad.

This season I hired an electric needle scaler, (air-powered and SDS accessory types are also available). It certainly seemed to do a good job, and I followed up with rust killer and Vinygard before antifouling, but I will have to wait until next liftout to see if it was more effective than my previous effort with a grinding disc to clean out deep pits. It now occurs to me that my use of the needle grinder would have had the same downside as using a wire brush. Can you comment , Vyv?

 

[vyv_cox]

This season I hired an electric needle scaler, (air-powered and SDS accessory types are also available). It certainly seemed to do a good job, and I followed up with rust killer and Vinygard before antifouling, but I will have to wait until next liftout to see if it was more effective than my previous effort with a grinding disc to clean out deep pits. It now occurs to me that my use of the needle grinder would have had the same downside as using a wire brush. Can you comment , Vyv?

Yes, needle guns are not ideal on cast iron, the surface has variable hardness due to the graphite flakes. The needles may penetrate the flakes and dislodge them, possibly forcing rust in. Needle guns are great on steel, used offshore it grit blasting is not possible.

 

[Graham376]

Don't bother with epoxy, unlikely to get it clean enough for it to stick without a complete blast. for smaller areas a grinder is probably best, although others favour flap wheels. once clean conventional primer such as Primocon and antifoul. If there are pits that annoy you then you can use an epoxy filler after the Primocon. With iron keels unless you blast I think you just have to accept that anything you do is just regular maintenance rather than a permanent fix.

Fertan should be available in Spanish DIY stores. I used that on rust spots then Jotun Vinyguard (IIRC made in Spain) followed by antifoul. Where I found holes due to sand casting defects, I used the Fertan and then car body filler, followed by Vinyguard, still OK after several years.

 

[alahol2]

There's little or no point in using rust converter if you have abraded back to shiny metal. It converts existing rust not cast iron or steel. I use Fertan every year on the few rust spots, I'm never sure if the same rust spots come back every year or if they're new ones...

 

[MikeCC]

For small areas, a polycarbide disk on a grinder is good. Gets a bit expensive for doing the whole thing.

I didn't use Fertan type product on mine. Two coats of zinc epoxy primer on bare metal, then filling/fairing. Two more of marine epoxy primer followed by antifoul.

Couple of spots broke through but nothing significant. Will wait to see what it looks like on lift-out after this second year.

 

[Spirit (of Glenans)]

There's little or no point in using rust converter if you have abraded back to shiny metal. It converts existing rust not cast iron or steel. I use Fertan every year on the few rust spots, I'm never sure if the same rust spots come back every year or if they're new ones...

The camera on your phone is a very useful tool ....

 

[alahol2]

The camera on your phone is a very useful tool ....

That's much too logical and far sighted! Extremely good idea though.
I've probably only ever taken 3 or 4 photos on my phone, never think of it. The technology has passed me by. I've never had my box brownie with me in the boatyard at the right moment.

 

[dankilb]

For small areas, a polycarbide disk on a grinder is good. Gets a bit expensive for doing the whole thing.

I didn't use Fertan type product on mine. Two coats of zinc epoxy primer on bare metal, then filling/fairing. Two more of marine epoxy primer followed by antifoul.

Couple of spots broke through but nothing significant. Will wait to see what it looks like on lift-out after this second year.

This would be my pick of the methods suggested above - if you want to try and prevent (rather than postpone) it coming back.

Our keel was blasted earlier in the refit and is currently coated in ZP epoxy primer awaiting some filling/fairing and further epoxy barrier coating.

The above quoted post would be my method of repair for our new coating system. But there would be less point doing this if the rest of the keel coating was unknown/unsound.

If the aim is just to keep it down for a year or two, converter and hull primer seems the way to go.

However, I’ve always doubted the adhesion qualities of Fertan (and indeed most rust converters). The black residue has always rubbed off on my hands, even after the suggested washing. I’d at least rub back and clean (degrease/solvent) well before priming.

Or use dilute phosphoric acid instead. I gave our keel a single wash (carefully brushed on - while in PPE) with a heavily diluted mix of some dubious stuff I found in a locker (and so I claim no expertise and do your own risk assessment…!). But we found that it ‘converted’ the oxidation so well, after grinding, it stained quite deep into the substrate (the blasting operative subsequently cursed it because it stained black and made it harder to see where he’d been!).

 

[dankilb]

I found one of these (£5.50) would do an area maybe max half m2 of heavy rust and 1m of surface rust/paint:
Non-Woven Preparation Wheel 115mm

…that’s until I gave up and had it blasted at the same time as part of a subsequent osmosis investigation/treatment!

Still cheap compared to hiring kit and no risk of grinding in metal particles from wire brushes. I found it mostly coarse enough to dislodge, rather than just polish up and hide, nasty flakes or pocks of rust. Definitely better than a flap disc for this . But you do need to move the disc around a lot - and smash through them heavy-handidly! - to have the best chance of getting out all the rot.

 

[Spirit (of Glenans)]

That's much too logical and far sighted! Extremely good idea though.
I've probably only ever taken 3 or 4 photos on my phone, never think of it. The technology has passed me by. I've never had my box brownie with me in the boatyard at the right moment.

I'm worried now that I might be causing deterioration of my memory, as I no longer have to remember things; meter readings, prices of stuff in shops that I might want to compare etc etc, just take a photo!