Cast iron keel coating?

We are having a keel fin cast in iron. We will be encapsulating the iron fin in a fiberglass skin.

Any guidance on what to coat/treat the iron with protect it under the skin?

Sam

 

Grind the surface of the fin clean with 100 - 120 grit paper, wipe immediately with a acetone damp towel, let it flash off, then immediately apply straight, unthickened epoxy. If you work "wet on wet" and apply additional coats of epoxy, you can get the fabric on with a chemical bond (which is always best). I'd recommend a minimum of three base coats of epoxy, followed by the fabric. Wait until the third coat is just barely tacky or has just lost it's tackiness, before applying fabric layers.

If you use a different resin then epoxy, eventually moisture will get to the keel, cause it to rust and pop off the sheathing.

 

PAR’s advice is right on. You really want to get something on the keel fast after the keel is "shiny."

 

new keel

Sam, where are you having your new keel poured? why do you feel you need to lay a fg skin on the CI?. CI craks most likel;y due to stresses left in during the cool of period after pouring or, conceivavble bumping into something though I am not sure that the stresses are large enough to crack CI. any naval / structural talent out there?

 

We are getting the keel cast in Floresville, TX.

We use a fg skin to reduce the weight of the fin to allow few more pounds in the bulb and provide a fair surface to the water.

Seem we are getting the most surface rust near the attachment point of the fin to the hull. Almost impossible to keep the water out of the area.

Any thoughts?

Sam

 

What about deep pitting? Any suggestions regarding sandblasting?

 

I would heat the thing really good with a blow torch on a low-humidity afternoon...wait till the keel cools....then follow the Naval architect( formerly known as PAR's) advice to the letter....I think dry season is the best time to do this...if you can find a good sunny 2-3 day window of low-humidity,highpressure,and no rain or fog...

 

Just a thought - would it be economical to 'tin' the steel, That is, heat the iron up then put a layer of solder on to form a thin, flexible, rust proof layer over the steel, before other coatings ? Same principle as soldering copper roofing. It works well on copper, but I have no idea how it would work with cast iron.

Depending on your motivation, you could probably coat the cast iron with copper, using electrolosys like the experiments in science class. Sit the iron in a bath of copper sulphate, and apply a weak voltage from a copper Cathode through the iron keel.

Even a good epoxy is going to let some moisture in over time, so a metallic solution might be the go.

 

The vast majority of keels in the UK are cast iron. Millions of man hours have been spent making them 'rust free'.

I don't know anything that has worked in the long term. Everyone here seems to be resigned to them being 'an opportunity' for regular maintenance.

Unlike steel, cast iron always seems to have inclusions which means you can never blasy back to a surface that would class as SA2.5. Therefore any coating, however carefully applied, has a much shorter life than hoped. But dry blasting till it looks bright on a warm dry day, immediate coating with west epoxy, fairing with made up west system putty, then several coats of VC Tar and then VC17 antifouling, seems to keep my race boat keel as smooth and rust free as any other cast iron keel I have seen. Still needs touching up each year and completely redoing in no more than every 10 years.

 

Crag, what dry medium do you suggest?

 

Sand is fine, but over here, more and more boat yards are restricting the use of dry sand blasting because of the mess. However the alternative 'wet' approaches will have the cast iron flash rust before you can get it dry.