zinc based primer on keel - cast iron?

[lancelot]

Should a zinc based primer be used on a cast iron keel?
I need to antifoul the yacht,s bottom and keel. Before I do, I need to get rid of some rust spots. I am only out for a couple of weeks, so time is short. I intend to wire brush the small areas and spots, use a rust converter or gel, primer and antifoul. I am not interested at the moment in using a resin based solution. Just a quick repair. Would a primer incorporating zinc be feasible or not?
Fair winds,
Lancelot

 

[jwilson]

Well it's what I do every year on the rust patches on my iron keel.

 

[theoldsalt]

Same with me. I use Kurust from Halfords as it is water based. My preference for primer is Hempel underwater primer (several coats) as I get much better protection than with primocon.

 

[TQA]

Needle gun to shift the rust is much better than wire brush or grinder.

A phosphoric acid based rust converter. I use Ospho.

A zinc rich epoxy primer. I use the one made by Sherwin Williams.

 

[Robin]

Should a zinc based primer be used on a cast iron keel?
I need to antifoul the yacht,s bottom and keel. Before I do, I need to get rid of some rust spots. I am only out for a couple of weeks, so time is short. I intend to wire brush the small areas and spots, use a rust converter or gel, primer and antifoul. I am not interested at the moment in using a resin based solution. Just a quick repair. Would a primer incorporating zinc be feasible or not?
Fair winds,
Lancelot

International Paints 'Primocon', is easy to apply, dries fast and is a primer for their antifoul paints as well as a barrier coat to use between an unknown bottom paint and a new one. I'm a year or two out of touch with UK products but Primocon was certainly the preferred product and worked for me, I assume it is still available ora similar/updated version of it. I used 'Fertan' or similar to kill rust first after wire brushing and course sanding, then IIRC wiped it over with meths to remove any excess/undried bits before applying the Primocon..

 

[vyv_cox]

Cast iron is a mixture of iron, iron carbides and free graphite flakes, which are soft and rather porous. Wire brushing tends to drive rust into this natural porosity. Needle gunning is great on steel but less so on cast iron, whose surface hardness varies considerably. Angle grinding is probably the most effective method if grit blasting is not used.

 

[Halo]

You will not stop the rust. Zinc primer may help a little but you will still be on an annual programme to keep rust in check. No problem with this as folk have done it for decades

 

[Seajet]

I have had a boat with a mild steel keelplate and cast iron ballast bulb for 39 years, during which time I've tried every coating going - I'm a qualified engineer - but nothing like Vyv - and have also temped in a chandlery with access to most things going for yotties, but tried all sorts.

I'm now on my second keel as I wasn't clued up enough as a teenager...

Having tried everything going, inc zinc rich Galvafroid, all the patent stuff, 2 part epoxy - which when inevitably cracked traps water - etc, I came across

Dulux Metalshield.

It's one part, but does require the special one part primer on bare metal.

I know ' Dulux ' sounds like a joke, but this stuff is brilliant, it's the only treatment I've ever tried which looks the same at the end of the season.

 

[ghostlymoron]

I tried Hammerite special metal primer one year on my cast iron keels which seemed to work pretty well but as I no longer have that boat, I can't say if it lasts more than 10 years, but it seemed better than most stuff. I came across a test piece I did at the time (2005) that has survived well in my garden.
In my experience, you'll never completely stop rust patches using self applied remedies. You just have to accept an annual brush/grind/coat régime - it's cosmetic after all and will never reduce the mass of the keel to a significant extent.

 

[KellysEye]

Don't use a wrire brush it leaves small bits of wire behind, rust converters don't work the rust comes back. The only way to stop rust is get back to bare metal, if the areas are small you could use a dremel grinder and make absolutely sure every tiny bit of rust has gone. Then coat with Zinc rich epoxy paint. If there is high humidity do one bit at a time then paint otherwise you risk flash rusting. I learnt this from owning a steel boat.

 

[ghostlymoron]

Steel is very different from cast iron as Vyv points out. The latter contains FAR more impurities and inclusions so you'll never get back to 'bare metal's. This is why, whatever you do, it'll need repeating in the future.

Don't use a wrire brush it leaves small bits of wire behind, rust converters don't work the rust comes back. The only way to stop rust is get back to bare metal, if the areas are small you could use a dremel grinder and make absolutely sure every tiny bit of rust has gone. Then coat with Zinc rich epoxy paint. If there is high humidity do one bit at a time then paint otherwise you risk flash rusting. I learnt this from owning a steel boat.

 

[rudolph_hart]

Did my cast iron wing keel in March 2009 & all I've needed to do since was antifoul it!

I'd post a piccie, but the Forum software is as hostile as a cornered rat (why not right click on photo, select Copy, return to text, right click, select Paste - oh! it's greyed out, wot a shame):livid:

Angle grinder, first with disc, then wire brush to bare metal. Rust Converter over the whole keel. Fill any holes with epoxy filler. 2x coats of Jotun Jotamastic Aluminium Primer. Sand smooth. Antifouling Primer. Antifoul.

Bob Sherunkel! :encouragement:

 

[Seajet]

Hammerite was one of my earlier tries on my cast iron ballast bulb; it lasted about as long as toothpaste.

I understand it was changed a while ago, may possibly have been useful about the times depicted in ' Heartbeat ' but useless now.

Metalshield gives quite a hard, tough but not brittle coating.

 

[ghostlymoron]

Ah but I was talking about the Special Metals Primer.

 

[Aeolus]

I take my boat out of the water every other year. I treat the cast iron keel with vactan rust converter, then teamac metaclor primer and finally two coats of antifoul. Rarely much weed even at the end of the second year and very little rust on the keel.

 

[lancelot]

Thanks everyone, for the advice. I realise the keel needs frequent maintenace. I will try the Dulux idea next time.
Fair winds,
lancelot

 

[GrahamHR]

Wouldn't such isron based keels benefit from an applied potential corrosion protection system ? Like the oil rigs use. Volvo Penta have one for their aluminium outdrives, I'd imagine they'd prrotect a steel keel forever.

 

[rudolph_hart]

Thanks everyone, for the advice. I realise the keel needs frequent maintenace. I will try the Dulux idea next time.
Fair winds,
lancelot

I think you may have misssed my point in my post (No 12). Since I did mine, it hasn't needed any maintenance in 7 years.

 

[PeteCooper]

Having seen Dulux Metalshield recommended on here and had a cast iron keel to prep I contacted Dulux to ask if it was suitable. They replied with a very definite no, and even went as far as saying that none of their products were suitable.
On the strength of that I used a flapwheel to get the keel back to as near bare metal as I could and then applied Primocon - I think that 4 coats was what the instructions advised but however many it was that is what I applied. I then antifouled over that ... and then sold the boat. She is still sat in the car park over a year later so I don't know how it would have fared in the water.

 

[KellysEye]

>Steel is very different from cast iron as Vyv points out. The latter contains FAR more impurities and inclusions so you'll never get back to 'bare metal's. This is why, whatever you do, it'll need repeating in the future.

I would suggest the OP tries what I've said, ensuring there is absoutely no rust use a magnifying glass should do it. And then reports back in a year about any returning rust, it has to be worth a try and I'm intrigued if it works. If it does there will be a lot of happy people here.