Cleaning Rust Off Cast Iron

My preferred method of rust stripping is a wire cup brush in an angle grinder, such as:

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.

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.

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.

Or this type...

or these:

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

Also, sheet the job up well or you could be in for some very expensive claims from surrounding boats with damaged gel coat.

Graham.

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.

A needle/descaling gun will do the job well. You can probably hire one, but don't forget the ear defenders

and a fairly hefty compressor - a needle gun uses a surprising amount of air if you use it continuously.

Floatything

Have it sandblasted. Remove it if needed. this way you will have all the loose material gone and the cleanest possible surface to work with. You can be satisfied your fillers and paints adhere properly with this method.

A needle gun with a compressor would be my 2nd choice. These are great for removing lumps of rust. Though , you would still need to use a grinder and wire brush with this method.

DP

A Hilti TE104 is an electric needle gun, works very well.....

And how much will it cost to hire this gun? bearing in mind the op is looking for the cheapest reliable method.

-- Sir Benjamin Middlethwaite

Unquestionably shotblasting as others have said, you will get the whole boat cleaned and primed in a day. However if you have a few weeks /months free time and wish to spend a similar amount of money using an angle grinder (disks at £5 each) you could take pictures and claim an ididit diy award.

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.

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?

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.

Shipyards use needle guns for descaling - whether it's worth hiring one depends on the size of the job and how much you value your time.