Rust reformer on cast iron - will epoxy stick?

[Northbound44]

Yes Epoxy will stick and protect the cast iron keel. However, the antifooling will not hold on epoxy. Better to prime the epoxy coating with Interprotec 2000 following by the antifooling as per manufacturer instructions. I did it on a Beneteau keel with good results.

[hpeer]

Wotname
Thanks for reply.

[Bycrick]

The Corrostop is more than just phosphoric acid. It also contains something that deliberately leaves a film on the metal to prevent new rusting. This film won’t wash off. I’m working on engine parts with it right now. Paint over the dried Corrostop with a good primer then apply your finish coats.

[gwyn]

Hi. I did my cast iron keel at the first opportunity I had. Completely ground down to bare metal. Leave to get a surace film of rust. Five coats of rust cure/preventer. Key surface. One coat of epoxy. Encapsulate whole keel in epoxy plaster. Two coats of epoxy with barrier powder added. One coat of epoxy with copper powder. Two coats (three on leading edges) of copper antifoul. Has lasted 12 years so far with very minor repairs.

[Goodxcharly]

Out of the blue! A thought. Could you have a few zinc strips welded to the keel. Seems like that would limit your corrosion and get you by till your next haul out.

I know that is what they do for steel work boats, tug etc.

[Jon Hacking]

Originally Posted by Northbound44

Yes Epoxy will stick and protect the cast iron keel. However, the antifooling will not hold on epoxy. Better to prime the epoxy coating with Interprotec 2000 following by the antifooling as per manufacturer instructions...

I beg to differ. We've been running antifouling directly applied to (lightly sanded) epoxy for MANY years with no issues at all.

But further to the OP's original question, we take a different approach to any presented so far.

To get epoxy to stick to ANY metal surface, we sand, clean, & dry the metal, then apply the epoxy, & immediately SAND through the wet epoxy with about an 80-grit wet & dry paper. This removes any oxide layer (that epoxy doesn't like) & prevents further oxidation, since there's no free oxygen now. This promotes a good chemical bond, & provides an additional mechanical key as well.

Additional epoxy layers should be applied wet-on-wet, when the underlying layer is just starting to kick (still a bit tacky) & without any sanding (either before or after) until the desired thickness is achieved.

While I must admit that I haven't tried this with cast iron, we've done this with our aluminum sail-drives for years, & aluminum is notoriously difficult to bond to, so I'd think this would work well on cast iron as well. It's also the only way we've found to get epoxy to bond to stainless steel.

[RaymondR]

You'd probably have been better off leaving the rust converters in the bottle and just sanding it off and applying a couple of coats of one of the really thin, low viscosity epoxy primers which will penetrate into the porous cast iron surface thereby getting good adhesion for later coats.

[Wotname]

Originally Posted by Jon Hacking

I beg to differ. We've been running antifouling directly applied to (lightly sanded) epoxy for MANY years with no issues at all.

But further to the OP's original question, we take a different approach to any presented so far.

To get epoxy to stick to ANY metal surface, we sand, clean, & dry the metal, then apply the epoxy, & immediately SAND through the wet epoxy with about an 80-grit wet & dry paper. This removes any oxide layer (that epoxy doesn't like) & prevents further oxidation, since there's no free oxygen now. This promotes a good chemical bond, & provides an additional mechanical key as well.

Additional epoxy layers should be applied wet-on-wet, when the underlying layer is just starting to kick (still a bit tacky) & without any sanding (either before or after) until the desired thickness is achieved.

While I must admit that I haven't tried this with cast iron, we've done this with our aluminum sail-drives for years, & aluminum is notoriously difficult to bond to, so I'd think this would work well on cast iron as well. It's also the only way we've found to get epoxy to bond to stainless steel.

I agree this method of wet sanding through the epoxy is the only way to get epoxy to stick to Al for the reasons given. A clean Al surface oxidises within milliseconds of being exposed to air.

I guess it can't hurt to do it on cast iron but I don't think it will help much and the effort isn't worth it. Depending on temperature and humidity, cast iron will take a few hours before it oxidises and perhaps more importantly, any wet sanding will only key the top surface. The lower (porous surfaces) won't be reached by the sandpaper grit.

Maybe one day I'll experiment on a cast iron engine block.

[BBill]

All these coating methods sound too complicated. What has worked for me on cast iron keels is grinder or drill wire brushing and cold tar epoxy but never any treatment or other barrier. Sand blasting is better on rough keels but not necessary if 4-5 yrs between treatments is ok. Bottom paints no problem as is. Regular epoxy resin should work but the thicker cold tar epoxy is better. No matter what method, if your cast iron keel doesn't get a spot or two bleed throughs or scrapes you probably aren't sailing much.

On second thought I've used red lead to spot treat.

[Redline452]

Originally Posted by Jon Hacking

To get epoxy to stick to ANY metal surface, we sand, clean, & dry the metal, then apply the epoxy, & immediately SAND through the wet epoxy with about an 80-grit wet & dry paper.

+1 on this. After a previous boatyard broke the after tip off my keel when blocking the boat, and told me I'd never get the broken bit to stick on, I took the West System approach of wire-brushing through the wet epoxy and the repair has held up perfectly. The broken surface was very irregular and sanding would not have been practical, but a coarse wire brush did the job.

[RaymondR]

Coal tar epoxy is a good product but is generally too thick to get any penetration. A thin etch primer first will penetrate and achieve better adhesion onto which the coal tar can bond well. Put the coal tar on when the etch primer is still tacky.

[Cheechako]

Originally Posted by Northbound44

Yes Epoxy will stick and protect the cast iron keel. However, the antifooling will not hold on epoxy. Better to prime the epoxy coating with Interprotec 2000 following by the antifooling as per manufacturer instructions. I did it on a Beneteau keel with good results.

Yea, antifouling holds onto epoxy, even just plain epoxy resin. You have to paint while the bond is still tacky or sand well before applying it. I have done both ways.
Whether the epoxy will stick well to the casts iron is another thing though!

The small areas I have tried "rust converter" on were a waste of time.
The problem is the item continues to rust under the hard surface created and eventually just bubbles up. Probably because you just cannot get all oxide or dampness out prior to using the converter...... and if you could.... why would you need the convertor?