Porous Cast Iron Keel

FulmarJeddo

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I have completely stripped all the anti foul from the hull this winter. Last weekend I stripped the keel back to bare metal. Once I got all the paint and rust off the keel appeared in remarkably good condition. I gave it a coat of Hammerite. When I went back this weekend I found the surface of the paint covered in small blisters. It seems that that there must have been moisture in the keel. I know cast iron is made of all the old rubbish, but is it possible that moisture is trapped in it? I have sanded the surface which seems to have burst the blisters. With time running out I am going to have to carry on and anti foul over it, but I guess the rusting will start all over again. The only thing I can see to do is, next winter strip the keel back as soon as the boat comes out of the water, cover it in some way to keep the weather off, but keep air flowing over it and hope that it dries out.

Anyone got any thoughts on this?
 

NorthUp

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Hammerite is well known for reacting with different types of paint- could there have been traces of old paint remaining in the pits and cavities of the keel?
Your next problem might be the Hammerite reacting with the antifouling !
 

Ronald_M

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Sorry to hear of your difficulty - I've recently had my keels slurry-blasted, and am in the process of repainting them. It's a hard, mucky job and not something one would want to do too often!

You're probably right in your guess that moisture is lodged within the keel - cast iron is very porous. Before your next attempt, make sure that the keel is stripped back thoroughly and is degreased carefully in order to provide the best possible adhesion for the new paint. Allow the keels to dry out completely, using gentle heat (hair dryer?) if possible. A note of warning: beware of the fact that moisture can be absorbed by bare cast-iron and freeze. Shattered keels are not unknown!

FWIW I have decided to go with the POR-15 system. If the manufacturer's claims are to be believed this should provide a durable long-term solution - I'll let you know! - but there are, of course, other methods - do a forum search for "rusty keels" or similar.

(Information on POR-15 can be found at:
www.frost.co.uk and www.por15.com )

Good luck,

Ron.
 

boguing

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I'm not going to be able to tell you how to deal with it, but can give you some very relevant background info.

I worked for MG, and may well have sold your boat to it's first owner.

I gave away a Spring 25 on the ITV Telethon in 1988. Winner had to guess the weight of the keel. To the nearest gram.

Being a scientific sort of a bloke, I asked the National Physical Laboratory to weigh it for me. They agreed, subject to it being painted with primer, and still having a post-casting residual temperature of 50 degrees Celsius. The Army picked it up from Cornwall and delivered it to them.

Apparently, cast iron will pick up 1% by weight of water within hours of cooling.

So, many years later, you have a damp keel.

With little knowledge of modern paints, I would remove the keel, grind it back, heat it to more than 100C for a day or two, and then epoxy it as soon as it was cool enough.

Probably overkill, but it would fix it. Good luck on getting the keel off. The mastic used was vv sticky. We had one boat hanging in slings all night, with the keel bolts all removed, and still had to use wedges to split it off the following morning.

Actually though, I'd do what we at MG really wanted to do, which was to fit a really deep lead-bulbed fin, and try and better the 15.4 Kts best speed that we saw.
 

FulmarJeddo

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Thanks all for the replies. I don't thick it is a reaction to existing paint as it seems to be rusty water in the blisters. The hammerite was to give a bit of extra protection. I will be applying Primacom next, hopefully next weekend. Have to see what it's like when the boat comes out of the water in the autumn. I don't think I will be changing the keel, the shallow wing is great for the east cost.
Thanks again.
David
 

NorthUp

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Rusty water it is then!
You could try building a tent round the keel with a tarp, ( including the ground) and run a dehumidifier inside for a few days, might be easier than heating a big lump of iron outside- or maybe combine the two, heat first then dehumidify?
 

Pasarell

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Unless you washed the keel very well after stripping old paint off I would look at salt as the primary cause of blistering.
All the comments about cast iron being porous are true and trapped water would not help of course but if you ground the old paint off on a dry day the heat generated would have caused much of the water to evaporate out leaving salt crystals behind. Solvent from the Hammerite would then be absorbed to cause the bistering.
I would steer clear of Hammerite like the plague - it's not designed for underwater use.
If you have time grind all the paint off again and pressure wash several times to remove salt from all the poores and pits in the iron, then coat with a zinc rich primer suitable for underwater use. A final quick grind to remove surface corrosion after washing. Build up film thickness with an intermediate waterproof primer (Primocon is OK but not particularly high performance) then antifoul.
You will never completely remove the corrosion with cast iron but grinding will do a pretty good job. Removing salt will be far more important in preventing blistering
 

mbay

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Having done my keel a couple of times and read all the keel posts on here IMHO you can't stop rust coming through again at some point in time.....how long in time depends on your budget and time.
 

paulwitney

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When I first bought my boat the cast iron keel was a mess. Big blisters of soft rust and very uneven. First slip tried sanding/grinding back and using rust converters, some epoxy filler for the biggest holes and Primacon followed by antifoul. Was definitely better next slip but still not too good. Tried same again with slight improvement. Last slip tried marine penetrol prior to Primacon and then antifoul. It has been over 12 months now and when I dived on the keel to give it a quick wipe down prior to a race last Monday, it looks remarkably better. There are still some small bubbles of rust coming through but nothing like before! The only problem I have now when diving under the boat is the thought of big nasty grey things swimming by!!! Three shark attacks in three weeks around Sydney puts some very negative thoughts into your mind, especially when the water is a bit murky!
 
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