Painting rusty cast iron

[Seagreen]

Yes, there was a post about not so long ago, but the new search engine didn't reveal it.

So what can I paint an old cast iron keel in, to stabilise it enough;
A) to take some fairing compound and
B) to let it hold red oxide primer to hold the antifoul?

Or maybe red oxide paint was the answer all along? It's a very rough casting with some large voids which need filling, and have old barnacle shells in. I want to stabilise the rust first. The bottom edge of it looks like an extra from a Jacques Cousteau movie and i think I'm looking for large quantities of "Jenolite" or similar. I'd like to get it smooth and shiny.

 

[Billy_Mizzen]

Coincidentally I’ve just done all Billy Mizzen’s underwater Ironwork, firstly I used a 3 ½ “ grinder, fitted with a coarse-twist wire brush and blasted away until I got off all loose rust and back to clean shiny metal.

For all the bits adjacent to timber or in the nooks and crannies the grinder wouldn’t get too, I used a chipping hammer, wire brush and various odd bits of sandpaper!

When this was done I gave it all a coat of Jenolite, then five coats of Red Oxide primer!

 

[raven]

I can recommend a product called neutrarust http://www.neutrarustuk.com/ which I have used successfully on my steel hulled boat.

 

[Seagreen]

Ok so now here's the dilemma. A bit of googling reveals that both one litre of Nutrarust and Jenoseel both sell for £19.99. Tactically, I'd only need to briefly brush the keel and add a couple of coats of Nutrarust and then its "sorted" if it does "exactly what it says on the tin" - I wouldn't have to grind back to shiny, which may be impossible anyway. Which do I go for? Anyone tried Jenoseel?

 

[ccscott49]

I know jenolite, which is good stuff. We use some stuff out here on the rig and for the life of me I cant remember the name of it! Some acid or other "Ospho", thats the stuff, phosphoric acid based, it's brilliant if you can get hold of some.
For those barnacles in the hollows, use hydrochloric acid or vinegar to dissolve the shells, then winkle them out, got to get those holes clean as possible, before rust killing and filling. Hammerite do an excellent rust killer primer, called No 1, its very good stuff. But any metal epoxy primer would do, coal tar epoxy is my bruvs favourite.

 

[oldharry]

Hire a needle gun from the local hire shop. Its the most painless method of removing loose rust and scale. 'Rust convertors' based on phosphopric acid are OK, but do not give much protection in sea water, but to work properly any loose rust must first be removed, otherwise they do not reach the sound metal underneath. Some of us reckon they are a bit of a waste of money, because they dont last in seawater.

An angle grinder will appear to give a good bright finish, but actually does not. What it does is to smear the metal over the rust pockets which are then left untreated and waiting to pounce when the boat goes back in! The finish looks good but sadly - for all that work - 'tis but an illusion! Also they do not remove scale without a lot of hard work.

My last boat had plate steel skeg and bilge plates. Both were heavily rusted and scaled. I was advised that shot blasting was the only solution because of the degree of pitting, but that meant taking them all off - a major structural job in itself with the skeg. A car mechanic then suggested the needle gun - widely used on the motor trade for MOT work. End result: a good clean sound surface ready for rust proofing and fairing.

 

[Casey]

I used the method described by Billy Mizzen on Kala Sons's keel and then painted the cast iron keel with Blakes Keel Prime. Five years on it still looks good.

 

[Seagreen]

As I'v esome experience with Jenolite, I'm actually taking a punt on the Neutrarust, just to try something a bit different. The key thing, reading the blurb, is the savings in preparation time over the jenolite method. I plan two coats over brushed rust, filling to a smooth surface, priming, and then the antifoul. I'll report back on how well this has fared. I also don't plan on drying out for a year or so, so the base of the keels won't get ground into grit too often. We'll see if experience matches the blurb. Thanks for all the input.

 

[raven]

I should qualify that I have only used Neutrarust above the waterline. However I think that if you use it to neutralise the rust on the keel (just remove the loose surface rust but dont grind back to bare metal as the product actually needs the rust in order to make the chemical reaction) then prime and give a couple of coats of anti-foul you should be okay.

One work of caution - if the keel is showing signs of corrosion what are the setting / keel bolts like?

 

[aztec]

don't know if it's any help but i use hydochloric acid on rusty bare metal.

couple of applications brings it up like new. I'm lucky and have a mate who can get it easily, he also can get me HC with and anti rust additive to prevent the clean metal from oxidising...

not sure where you'd get it though, a good start would be your local chemical distributor, i don't know if there are any problems around supply, but you do need to follow any guidance on personal protection while using.

Iv'e also found that a good quality zinc based primer (like "Galvafroid") works very well as a substrate prior to filling and painting.

as i said, dunno if this helps. just an opinion.

steve.

 

[Seagreen]

I was thinking of doing just that. Two coats of the Neutrarust (I think that's all a litre will stretch to - Its a long narrow bar) then two pimers, some putty filler , more primer and then AF. Should keep out the barnacles and rust. My biggest problem is moving the wedges round to keep a 10 ton hull safe and still be able to paint all the keel.

 

[Peterduck]

Hydrochloric acid should be available at any reasonable builder's supplier, but it will be called 'Spirits of Salts'. It is used to clean cement from secondhand bricks. This is an archaic name harking back from the days of alchemy, I kid you not!
Peter

 

[ccscott49]

On the theme of old names for acid, Sulphuric was called "Oil of vitriol" due to it's oil like appearance in concentrated form, good eh!