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Rust sealer for steel keel

Jun 20, 2021
92
Beneteau 343 Fort Lauderdale
I'm finishing up sanding my bottom but the keel had some blisters and I sanded down to bare metal.

Any recommendations for a rust inhibitor/primer? plan to put on a gel coat next.
 
Jan 30, 2012
1,091
Nor'Sea 27 "Kiwanda" Portland/ Anacortes
On the keel - probably cast iron - consider an epoxy overcoat.

Read this for a start for some perspective: G/Flex Keel Repair - Epoxyworks

In any case a three coat epoxy undercoat - direct to the metal - would be the best choice on an iron keel - defects or no. Bottom paint goes over the (washed and scuffed) epoxy. No gel coat on the keel.

For that matter no gel coat anywhere else either unless there is a very good reason - like a visible repair that needs to be color matched.

Charles
 
Last edited:
Oct 24, 2010
2,399
Hunter 30 Everett, WA
First, as soon as you finish sanding on the keel, paint some OSPHO on it. It's really thin like water and only take seconds to paint on the keel. This chemically changes iron oxide into a protective layer that will prevent further rust. In my case with my cast iron keel I did that and then put three coats of barrier coat on top and then finished up with bottom paint. My keel has shown no evidence of rust since and it was relatively rough when I bought the boat in 2015. There is no sign of rust to date. OSPHO is really cheap and a quart will probably last you a lifetime. And you will find all kinds of places to use it.
 
Jan 30, 2012
1,091
Nor'Sea 27 "Kiwanda" Portland/ Anacortes
Ospho is an orthophosphoric acid solution. The value of Ospho is that it converts iron oxides (rust) to iron phosphate which is pretty inert. However, unless neutralized the treated surface will remain acidic. There have been considerable number of adhesion failures with polyester, epoxy, epoxy primers (barrier coatings) over Ospho treated metal. These will resemble large blisters. Adhesion problems can be eliminated. Let the treated metal nearly dry, then do a complete wetting and scrubbing of the treated area with clear water, then sand overall with 80 grit before applying any epoxy or epoxy primers. (See a discussion here: How to neutralize Ospho | Southern Polyurethanes Forum (spiuserforum.com)

My view (after suffering disappointments) is don't use acid treatments/cleaners (PPG, Ospho, POR 15) if the metal is pretty clean - which sanding with 80 will usually accomplish. All these treatments are converters - they do nothing to prevent future rust.

Charles
 
Last edited:
Oct 24, 2010
2,399
Hunter 30 Everett, WA
Ospho is an orthophosphoric acid solution. The value of Ospho is that it converts iron oxides (rust) to iron phosphate which is pretty inert. However, unless neutralized the treated surface will remain acidic. There have been considerable number of adhesion failures with polyester, epoxy, epoxy primers (barrier coatings) over Ospho treated metal. These will resemble large blisters. Adhesion problems can be eliminated. Let the treated metal nearly dry, then do a complete wetting and scrubbing of the treated area with clear water, then sand overall with 80 grit before applying any epoxy or epoxy primers. (See a discussion here: How to neutralize Ospho | Southern Polyurethanes Forum (spiuserforum.com)

My view (after suffering disappointments) is don't use acid treatments/cleaners (PPG, Ospho, POR 15) if the metal is pretty clean - which sanding with 80 will usually accomplish. All these treatments are converters - they do nothing to prevent future rust.

Charles
Interesting, but not my experience. My keel still looks great. I've used it on stabilizer jacks on my RV, a limber rack on my truck, and other places. I haven't seen any rust return. I do however give it a good rinse before painting.
Ken
 
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Jun 14, 2010
1,726
TBD Looking for my next boat CT
Coal Tar epoxy. both of my boats have cast iron keels. That's what always use. A warning-it stinks while uncured
I hate that stuff! (I don't use that word often).
It runs, sags, while setting up -- and it's impossible to sand smooth without gumming up the sandpaper in 30 seconds. If you put it on too think it cracks from contraction/expansion over the winter months, and water gets into the cracks to start the rust cycle again. Then it's back to removal (which should probably be done by abrasive blasting, but I always suffered through the gummed up 40-grit and wire brushes.