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
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?
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.
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
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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.
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.
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.
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.
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
Without getting too involved with technical explanations both are cutting
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
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.
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
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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