A proper job on iron keels: how best to proceed ?

Pirx

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Every year I bodge up the flaky, rusty patches on the cast iron bilge keels, and every year I promise myself that next time I'll do it properly .........

I'm very impressed with the 'rotary blaster' I have just used on the bad bits: it works easily, isn't too noisy and doesn't throw the mess everywhere. So I think I could get the keels back to bare metal without having a heart attack, and if I start early next winter ....

I'd really appreciate some advice about how to proceed once the keels are cleaned up.
 

vyv_cox

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I agree, those rotary blaster devices look to be very effective and probably do not drive rust into the metal, which with cast iron is easily done.

The standard advice is to do the job on a warm, dry day. Maybe not so easy in UK but a polythene tent would help the ambient conditions and prevent debris from being thrown far and wide. I would do one side at a time and aim to get an epoxy sealer on the bare metal within 20 minutes of cleaning. Coppercoat recommend a primer for this but I have used West quite effectively. Follow up with further coats, up to five, before the previous coat has fully cured, ideally still tacky.
 

Snoopy463

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I agree on the multiple coats of expoxy primer approach. I think we ended up with six or seven coats followed by coppercoat and problems have been minimal since.

Care to mention the brand of epoxy primer? I’m just about to go down the Coppercoat route and they recommend Gelshield 200 for the abraded gelcoat prior to Coppercoating. They didn’t say anything about steel keels and I neglected to ask.
 

Seajet

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In 41 years I have tried every treatment going on my mild steel keel plate and cast iron ballast bulb - the plate was originally galvanised but this scrapes off eventually.

I think epoxy was a mistake, as the slightest chip or crack trapped salt water against the metal; on a half tide mooring it's dry half the time, though trapped water seems somehow more corrosive than free flowing.

I had a new keel plate fitted a good 20 years ago, and now use Dulux Metalshield, the only stuff which looks much the same when the boat is hoisted out at end of season - it's one part but requires the special one part Metalshield primer, best applied to bare or galvanised metal, it can pickle other paints.

When my first keel corroded I think there were several other factors; for a start I didn't know any better so wintered her ashore with the keel retracted, unmaintained for the first few years.

Also, I later discovered ' Starfish 16 '

This was a decoy system of controllable fires on rafts and along the shoreline on Langstone Harbour to lure the Luftwaffe away from the vital Portsmouth docks, leading straight to our moorings - there are still circular patches of softer mud, bomb craters - and I think the tons of German steel and magnesium have an effect.

Other boats inc Andersons don't suffer but my boat is now the longest serving in the club.

When my first keel corroded I got in touch with Portsmouth Uni' Marine Metallurgy Unit, as I was told they were researcing corrosion.

They were eager to have a look, their pet theory of the time was microbes which excrete stuff with a severe corrosive effect on steel - apparently these are having a go on the wreck of the Titanic - but not so in my case.

The recently retired head boffin solemnly pronounced the deep pitting in the 3/4" steel was ' due to the speed of the boat through the water ' - even I wouldn't claim that !

I smiled politely and took the compliment...

Meanwhile, I cannot recommend Dulux Metalshield too highly; ' Dulux ' may sound like a joke but this is serious stuff.
 

lw395

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What you can get away with depends on how porous the iron is, and some cast iron is quite porous, particularly near the surface.
On an old boat, we once took the full-on approach.
Remove keel
flap disc to remove surface rust from keel
Phosphoric acid to stabilise rust in porous bits
Wash off acid
Take the keel indoors for 2 months to dry out
Lots of West epoxy
Fill and fair with more epoxy.

This worked very well, eventually the rust came back where the keel got scraped. It was a racing boat so playing in the shallows was pretty common. But just touching up the epoxy kept it smart for more than 5 years.
If we did it again, a few layers of sacrificial glass to take the abrasion might be called for.
The best way to deal with the rusty keel problem is to have a lead keel!

I think the key to doing a reasonable job each year is to get the keel as dry as possible and epoxy it when it's as warm as possible.
 

tross

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Yeah, Been there, done all the different coatings etc, etc.. and it was always waiting for me when it lifted out in the winter. Mine was worst at the join between the hull and the keel. Then one year with new toy - sorry tool, I decided to clean it down to bare metal with the dremel. What I found out was what looked like good iron was in fact, little pockets of rust! By using a little pionted grinding stone I was able to clean these out to new metal - This stopped 99% of the rust coming back each year.
 

john_morris_uk

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Care to mention the brand of epoxy primer? I’m just about to go down the Coppercoat route and they recommend Gelshield 200 for the abraded gelcoat prior to Coppercoating. They didn’t say anything about steel keels and I neglected to ask.

It wasn't gel shield but I don't remember the make. I got it through the club and it was from Blaster Master(I think I've got the name right) the keel shot or sand blasting person in Devon. Yoda of this parish will know as he helped organise the purchase.
 

stevd

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Sorry, I deleted my post instead of editing!!!
I tried a different approach this year, so I dont know if it will be a success.
I used a preparation wheel from screwfix on an angle grinder to remove the old paint and rust. Unlike a flap disk these dont grind the metal or create sparks.
Whilst this removed most of the rust, it will never be perfect. So after I wetted the keel to create a surface rust which I then converted using a product called Vactan (I think). This creates a primer layer too. I then painted with a zinc rich primer, and going to put 5 or 6 coats on. Time will tell
 

johnphilip

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I remember a guy who ran a boatyard on the East coast who, when asked which primer to use on keels would enquire what colour of primer the yachtsman would like to be scraping off in a couple of years time?
 

sailor211

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I took our keel back to bear metal, as I was not in a hurry I let it rust a little and washed it a couple of times to wash out any salt remained. I then took most of the rust off and coated with plenty of Phosphoric Acid to convert any rust, keep the metal wet with acid for 45 minutes. My tests showed that having done this no rust would reappear if left with no other treatment for weeks. I believe that this part is important. Remember rust convertors need rust to work so if you linish the metal, phosphoric acid or similar does nothing. If you do not treat any rust in porous metal it will reappear.

If you stop the rust, and exclude all moisture and air it wont rust again.

The main coating i used was Reactive Resins Armourguard epoxy. A flexible epoxy, with a high rubber content, designed for water tanks. The flexibility helps with movement and expansion. Reactive Resins are no longer, but flexible epoxy tank paints are and i believe worth considering,#. The build up was about 2mm, multiple coats to eliminate pin holes. After a final fairing, there was a couple of coats of totally solvent free epoxy resin, then coppercoat.

I wil tell you in 8 years if it last 10 years. So far so good.
#
 

rogerthebodger

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The damage I sustained on my steel boat had he surface rust removed with a heavy duty wire brush on an angle grinder then painted with hydrochloric acid followed with Phosphoric Acid. This was then followed with several coats of epoxy tar.

Then I build the boat The hull was grit blasted then painted below the waterline with epoxy tar.

Other than the damage sustained in the durban marina breakup no rust below the waterline. To me the mot important to to remove all the rust then coat the clean metal with a flexible epoxy.
 

yoda

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It wasn't gel shield but I don't remember the make. I got it through the club and it was from Blaster Master(I think I've got the name right) the keel shot or sand blasting person in Devon. Yoda of this parish will know as he helped organise the purchase.

The blasting was done by Moorland Blast Cleaning services. The epoxy primer was Hemples Hempadur. Total dry film thickness and good filling of any voids after the first coat are the important keys to a good finish and preventing new rust.

Yoda
 

Pasarell

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Most of the posts here sketch over preparation and focus on coating products. Surface preparation is absolutely key to coating cast iron and good preparation is very difficult to achieve. Also worth bearing in mind that most of the corrosion on cast iron keels occurs when the boat is out of the water - you just see it at lift out time! Also bear in mind that keels are rarely steel, they are cast iron so a different animal altogether.
Cast iron keels typically have lots of pits and voids in the surface so it is important to prepare those as well as possible. Without doubt dry abrasive blasting is the best preparation but if you want the best chance of success it is not enough on its own. Abrasive blasting forces salt crystals into the pits and holes in the surface and if they are coated over then you are just sealing next years rust into the surface.

Any preparation other than blasting is less effective, but needle guns come next with grinding a poor third. Wire brushing is pretty much a waste of time as it just polishes the rust and makes it look nice and shiny. Unless all corrosion products are removed from the cast iron they will be actively starting the process again as you apply your expensive epoxy.

The ideal process is to blast to remove old coatings, corrosion etc then pressure wash a few times to get salt and other debris out of the voids. When the iron is pretty much dry blast again (the process will heat the iron and help residual water evaporate from the voids). After the 2nd blast vacuum carefully to remove dust and loose particles. Don't worry about a little flash rusting as pretty much all suitable coatings are tolerant to that these days and getting rid of dust is far more important.

Coating choice is less important than ensuring enough is applied. Cast iron is typically a very rough surface with many nibs and pits. When applying by brush or roller as most of us do the peaks will get a very thin coat due to surface tension pulling the wet coating away from the high points. It is also very easy to bridge over the voids, especially with a roller, and trap air pockets which will blister and corrode. Best is to stipple the first couple of coats in with a brush to get into the voids.

It is really important to get enough coating over the nibs in the iron and the only way to achieve that is to put far more than recommended on the lower spots. Most coatings will give a minimum dry film thickness (dft) to be effective, that is the dft at the thinnest point. Don't achieve the dft and that is where the next rust spots will appear.

There are many good quality coatings to choose from and little to choose between them. A good zinc rich, solvent based, epoxy or urethane is best in my opinion and the name on the tin is not so important whichever of the marine coating suppliers it comes from. Pure epoxy is also good if the iron is prepared really well but since that is difficult to achieve the zinc helps a lot.

Rust converters have their place but not if you are trying to do a really good job. (There is a reason that none of the major marine coating manufacturers supply them). If the surface is prepared well there is no reason to use a converter. If the surface is not prepared so well and a converter is used it is essential to remove all the dry excess coating with a stiff brush and vacuum cleaner before subsequent coatings or adhesion will be compromised. And no converter is also an effective primer!

Obviously most of us don't get the opportunity to do the ultimate job but that was what the OP asked about!
 

lw395

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Pasarell makes some good points. I would add:
The 'nibs' or high points want grinding off. The ultimate job on a keel is a smooth keel.
Where there are pits or any other texture, the coating is in danger of being sanded thin at the edge of the pit. You want enough epoxy on there to make the minimum thickness adequate. This can be done by coating, filling, sanding smooth and then more coating.

Depending on the exact nature of the cast iron, the surface can be quite porous. As a result of this, I feel that drying is very important and rust conversion can have some value.
 

vyv_cox

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I disagree about needle gunning. Excellent on steel but a disaster on cast iron. It does not have the consistent surface strength to cope with this treatment and debris is driven in even worse than with wire brushing. Angle grinding is a reasonable second option after grit blasting.
 

Pirx

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Thank you everybody for all the information, especially Pasarell for the emphasis on proper preparation - which is the bit that I'm going to find difficult when parked in a damp field ...........

I'm beginning to think that the choice is between getting it right with a polythene tent, a portable generator and a heater or continuing with a policy of managed bodgery.
 
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