Cleaning Rust Off Cast Iron

I've a cast iron keel that has been rusting quietly under my boat for the last few years. Most of the antifouling paint on the keel is on rusty 'scabs'.
What is the best way of removing the paint and 'scabs' from the keel? Ways I've considered so far include a circular sanding device (in an electric drill) and an angle grinder.
Does anyone removed this type of rust before? What is the best way of removing it that doesn't take forever and cost a fortune in abrasive discs? Does on the those 'paint removers' with the spring steel tines work OK or do they just take off the rust?
Many thanks
Artie
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Arturo Ui wrote:

My preferred method of rust stripping is a wire cup brush in an angle grinder, such as: <http://www.screwfix.com/app/sfd/cat/pro.jsp?id 326&ts435>
--
Grunff

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You could always get it sand blasted professionally. It's not as costly you may fear, and it is quick and 100% effective (I had the fin keel of a 34' yacht done for £100). It is important though, as with all stripping back to bare metal, that you get a good coat of primer on straight away, as even a little bit of rust under the paint will create a weakness which will undermine any further coverings. It's another reason for getting the whole thing stripped in one quick go, as working on your own with sanders etc., often means leaving it for a while before priming.
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Grunff wrote:

Or this type... http://www.screwfix.com/app/sfd/cat/pro.jsp?cId 0424&ts980&id695 -- Sir Benjamin Middlethwaite
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The3rd Earl Of Derby wrote:

or these:
http://www.screwfix.com/app/sfd/cat/pro.jsp?cId 1282&ts728&id2538
Thoroughly recommended - both tough enough to do the job and, if used with care, gentle enough to feather paint edges.
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Pete Styles wrote:

I've used those, and while they do work reasonably well, they only last about 10 minutes in the grinder - not great value.
--
Grunff

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How did you get 10 minutes out of one? Catch an edge, or just some rough crusty rust, and they're worn out in 10 seconds.
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Andy Dingley wrote:

I got the whole of my keel (Bavaria 32 - quite a lot of rust) cleaned up with one and a bit, and I'd have thought that uner 10 quid a keel wasn't bad value.
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<snip>
Quite effective, but the brushes can be picked up much more cheaply than that - I usually stock up at autojumbles. Beware the flying brush bristles - they sting like hell, and you find them stuck in your clothes for days afterwards. These brushes can catch and kick back at you - keep a good hold of the angle grinder and don't get dangly bits of body or clothing too close.

Predictably useful advice: taking a keel to a bench grinder seems an odd way to do the job, to me.

Hmmm. Excellent devices, but perhaps not for this purpose. Great for large areas of paint, for example stripping a car bonnet, where their non-clogging and fine finish are an advantage, but, as Andy says, you can shred one in seconds on a thick rust scab or a sharp edge. Again, Screwfix aren't the cheapest (we're saying that a lot, these days, I think) - try a motor factors specialising in refinishing materials.
The OP doesn't say where the boat is, or how transportable it is, but commercial grit-blasting might be the best way forward, provided it can be immediately re-coated. It's even messier than using a cup brush, though, if the boat's in his living room (or vice versa).
Do any special precautions need to be taken with the dust from anti-fouling paint?
--
Kevin Poole
**Use current month and year to reply (e.g. snipped-for-privacy@mainbeam.co.uk)***
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I found that attacking the local defects / rusty 'scabs' with a proper hardened welders chipping hammer (the kind with a point and industrial quality) was very effective. The localised corrosion cells appeared to me, as if they might have been inclusions, voids or defects in the original casting some of which were roughly filled and faired at the time. Agree with others angle grinder with that cup brush is very effective for surface rust and a grinding disk for fettling any high spots. Automotive rust 'killers' such as "Kurust" seem effective at preventing recurrence and they convert any remaining rust to a black primed finish.

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Yip, phone your local undertaker
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Just a word of caution and something to be checked if using rotary brushes.
Blakes state that such brushes polish the metal and can cause adhesion problems for blacking. I know we are probably talking antifouling here but to be sure I think I would follow the brush up with a grinding disk to give the keel surface a key. This should be fairly fast and remove the worst of any surface corrosion that took place after rotary brushing.
--
Tony Brooks
www.TB-Training.co.uk
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On Thu, 12 Jan 2006 15:45:50 +0000, Tony Brooks wrote:

a grinding disk will do exactly the same thing!
its the raw speed at which the disk/brush passes over the metal that causes the polishing. A grinding disk may leave a scored surface, but inside the grooves the metal will still be polished (even more so)
You really need to use an etch primer to get a good bond after attacking metal with a grinder, this eats into the metal a little bit and anchors itself to any voids.
regards,
--
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http://maps.google.com/maps?q=ireland&llR.2711,-9.8630&spn=0.0244,0.0822&t=k
Cutting tools do not polish unless they are dull and rub. Wire brushes can be kept sharp
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How do you sharpen/keep sharp a wire brush. I have a both cup and dish? type wire brushes for my angle grinder, it has never ocurred to me their sharpness was under my control. Is the same true of grinding discs.
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On Fri, 13 Jan 2006 00:19:34 +0000, ChrisR wrote:

who said anything about cutting tools?
a grinder with a grinding disc is not a cutting tool
a grinder with a cup wire brush is not a cutting tool
a grinder with a sanding disc is not a cutting tool
For cutting, you would probably want some kind of saw. Please try to keep up.
--
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can
Without getting too involved with technical explanations both are cutting tools.
A proper hardened wire cup brush on an angle grinder can be sharpened and fettled on a grindstone by an experienced tradesman if it has been allowed to dull by rubbing. If you examine the end of an individual wire on an effective brush there will be a sharp corner at the end of each wire on an ineffective brush the end will be rounded and or bent over so there is no sharp edge. One problem with "as cast" finishes, cast iron keels for example, is that there will be a chilled (iron carbide) skin that is very hard and will dull a hardened brush very quickly reulting in rubbing. The brush is only really effective at removing material on the cast iron skin in this case. Rubbing the iron skin itself will not be terribly effective as it will merely glaze and polish. Depending on the grade of iron below the skin a soft grey iron is relative soft and machjineable, quite a different of course.
A grindstone or grinding disk consists of hard grit embadded in a softer usually polymeric matrix. The grit will be hard with randomly disposed sharp cutting edges. As the exposed sharp edges of the individual pieces of grit dull they are shed exposing new grits with sharp cuting edges. The composition of the wheel or disk must be matched to the material being ground if the wheel is not to glaze or clog. Lubricants canbe very effective to aid cutting and cooling some materials. A blunt brush or grinder is just as useless as a blunt knife.
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Electrolysis. Google for details. Works beautifully on cast iron.
This is usually done with small parts dunked in a bucket. However you can also make your own "bucket" from any waterproof bag you can arrange with thick polythene sheet. The electrolyte is messy rather than toxic, so spills aren't a disaster.
I've done the arse end of a narrowboat using this technique and half an old Microvax.
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Arturo Ui wrote:

How big is it?
If you can get it off, have it shot blasted. This might also be possible in situ.
If you want to de-rust it yourself, Grunff's angle grinder and cup brush suggestion is the way to do it. Wear overalls and eye protection (important!). N.B. using cup brushes in a weedy angle grinder will quickly shag it.
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Also, sheet the job up well or you could be in for some very expensive claims from surrounding boats with damaged gel coat.
Graham.
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