Cast iron keel touchup

[Sea Change]

What's the best performing, longest lasting, product to cover and protect small patches of bare metal on a cast iron keel?

No, I am not sandblasting it all back. That's not an option, for a variety of reasons.
No, I am not lifting out annually to patch small spots. Again, numerous reasons why.

There's a lot of snake oil around. I'm talking very small areas so it's almost a money-no-object situation.

 

[Chiara’s slave]

We find polyester resin has the best adhesion. Epoxy just doesn't stick as well. That drops off sometimes of it’s own accord, but polyester it takes another good knock on something hard to remove.

 

[geem]

From my past experience, we found epoxy was far better than polyester at bonding to cast iron. Its all about surface preparation. If you can remove all the rust then several layers of epoxy paint will do the job.

 

[Dellquay13]

Will epoxy stick to the surrounding old paint, as well as the bare metal? I thought you couldn’t get two pack to work over one pack coatings?

I just wire brush the few rust spots off and quickly get a few coats of hempel underwater primer onto the bare metal patches

 

[Chiara’s slave]

From my past experience, we found epoxy was far better than polyester at bonding to cast iron. Its all about surface preparation. If you can remove all the rust then several layers of epoxy paint will do the job.

Perhaps polyester works best for a quickie then. Which is usually the situation with us, it’s done while hanging on the crane. If it was left to the end of the season we’d be plumb last in every race.

 

[geem]

Will epoxy stick to the surrounding old paint, as well as the bare metal? I thought you couldn’t get two pack to work over one pack coatings?

I just wire brush the few rust spots off and quickly get a few coats of hempel underwater primer onto the bare metal patches

The idea is to epoxy the bare metal. Epoxy will stick to single pack paints but its generally a mechanical bond not chemical, like it is with epoxy onto bare metal.

 

[Roberto]

I just wire brush the few rust spots off and quickly get a few coats of hempel underwater primer onto the bare metal patches

I can make worse Quick wire brush, then direct antifouling over metal. It does not flake off, rust does not increase year over year, no barnacles nor growth. Obviously after 10y or so of such treatment the keel surface looks more like an orange skin than a racing foil

 

[vyv_cox]

My keel.was grit blasted, then epoxied with a couple of coats of clear West, then Coppercoated. Within the first mile of sailing I passed over a submerged float of polystyrene blocks cabled together with clips, which gouged a long scrape. Next winter I angle ground an area about 2 inches x 10, epoxied and Coppercoated with a small repair kit. It was well over 10 years before I needed to do anything further.

Wire brushing cast iron tends to push rust into the pores. I find angle grinding far more effective. Needle gunning is great on steel but even worse than wire brushing on cast iron.

 

[Chiara’s slave]

We use a battery angle grinder to get clean metal. Our issue is probably humidity, there’s 1.3 tons of wet wooden boat right above any keel repair. So keeping the repair completely dry is near impossible.

 

[Sea Change]

Looks like bog standard epoxy might be the ticket, then. Seeing as I already have plenty.
I'm using Seahawk Tuff Stuff epoxy primer on the gelcoat as a barrier layer, and they say it's suitable for bare cast iron too. That would be the simplest option.
I'm sure I've seen mention of primers with zinc or phosphoric acid in them, are these snake oil?

 

[Sea Change]

We use a battery angle grinder to get clean metal. Our issue is probably humidity, there’s 1.3 tons of wet wooden boat right above any keel repair. So keeping the repair completely dry is near impossible.

45% humidity today, 26degC.

 

[johnalison]

Some thirty years ago I had this problem and solved it with Hammerite used on the patches and over two or three years there remained almost no rust. I gather that today's stuff doesn't do the same, but there must be some substitute that does the job in a similar way

 

[Chiara’s slave]

45% humidity today, 26degC.

Excellent, but work fast

 

[DownWest]

Some thirty years ago I had this problem and solved it with Hammerite used on the patches and over two or three years there remained almost no rust. I gather that today's stuff doesn't do the same, but there must be some substitute that does the job in a similar way

Hammerite is a shadow of it's former self. Mainly because of the nasty, but effective, elements in its previous form.

I have had some success with a locally made epoxy primer, but I think the outfit had closed down.

The title of the thread made me chuckle.
My wife, from Washington DC, was not aware of some of the English expressions, so on an early drinks do, when we had several UK friends over. she would ask if they would like a 'touch up', meaning a refill of their drink...
We still laugh about it.

 

[vyv_cox]

I'm sure I've seen mention of primers with zinc or phosphoric acid in them, are these snake oil?

These primers are mostly single pack alkyds, solvent based, not great performers when two pack epoxies are applied on top. Etch primers, water based, can work well but are normally used on non-ferrous substrates. I have used the Hammerite version on manganese bronze (brass) beneath Coppercoat. Worked well.

 

[Sea Change]

These primers are mostly single pack alkyds, solvent based, not great performers when two pack epoxies are applied on top. Etch primers, water based, can work well but are normally used on non-ferrous substrates. I have used the Hammerite version on manganese bronze (brass) beneath Coppercoat. Worked well.

Thanks Vyv!