Hope you're hanging in against the forces of Mother Nature. Just had a
subscriber call asking about how best to treat rust on his Cal 20 keel.
His was the second call I've had on the topic. Wondered if you might
consider a Capt'n Pauley column on rusty keels. This last caller said,
"Do you think Paul would consider it?"
Well, it just so happens that my daughter and her friend just completed a keel job on their boat. It’s a little bigger than the Cal 20, but the principles are the same. Here goes!
First, why is rust a problem? Rust, ferric oxide, takes up more space than the native iron it oxidizes. That means that the rust flakes are constantly expanding and falling away, exposing more iron that in turn oxidizes and, well, you get the picture. If the rust forms through a pinhole in the protective paint, it will proceed to expand and flake off the paint.
Other metals, aluminum in particular, react differently. Aluminum oxide forms a tough coating that slows down further corrosion in most cases. COR-TEN (R) steel is also formulated to do the same thing, but none of our keels were fabricated from COR-TEN (R). Protecting a keel from rust can be done with proper preparation and application of protective coatings.
So why use cast iron instead of lead for keels? Foundries that can pour cast iron are much more prevalent than lead smelters. Lead also carries with it a heavy burden of environmental concerns. Lead is toxic and the fumes from melting it need to be carefully contained and monitored. Lead workers have their blood tested regularly for signs of lead poisoning.
Obviously, the first step in refinishing the keel is to get all the old rust off and prepare the surface for the protective coatings. There are several ways to accomplish this, none of them particularly pleasant.
In most cases, the keel hasn’t rusted uniformly. There are sound areas and areas where rust has made inroads, leaving pits. If you don’t get all of the rust off the iron and out of these pits, you will be forever doomed to redoing the job.
Knock off all the loose rust, scale and flaking paint with a grinder equipped with a coarse disc. By coarse grit, I mean something along the lines of 36-grit, real gravel. BE CAREFUL! Nothing takes off skin like a 36-grit disc (don’t ask how I know). Once the worst of it is off, you have several choices.
The first is to sandblast the keel. This takes specialized equipment and a great deal of trouble to contain the dust, but it is most effective. A second way is to mechanically remove the rust with something called a “needle gun”.
A needle gun consists of a set of steel needles that are driven against a surface, either by compressed air or by electric drive. The individual needles do a good job of getting into the pits and roughing up the surface with a good “tooth” for the coatings. These guns can usually be rented from a tool rental shop.
Failing either of those methods, its back to the angle grinder and discs for doing the bulk of the keel surface. A smaller Dremel (R) tool or die grinder will probably be required to clean out the pits and crevices.
This next step is very important. Cast iron can start rusting within an hour or two after being cleaned. Don’t waste any time in getting the first protective coating on the keel. To wait is to invite future outbreaks of rust.
So, get a coating on as soon as possible after getting the rust off. The first coating can be a compound like OSPHO. OSPHO, and other compounds like it, is a rust converter. It contains metal salts and phosphoric acid. Brush on the thin liquid and it immediately starts converting any remaining rust into ferric phosphate, a tough gray surface coating. The ferric phosphate also provides a tough surface that is an excellent base for the rest of the protective coatings.
After the rust converter, apply several coats of a metal primer. Most major marine paint suppliers, such as Interlux or Pettit, have their own range of products. They all work well but I like to pick one manufacturer and stick with all their products on a particular job. Mixing brands just never seems to work well.
With the bare cast iron surfaces protected from further rusting, you can contemplate the next step. If your keel has extensive pitting you may want to spend some time fairing it, or making it smooth. This is a job for an epoxy resin mixed with fillers. The fillers that sand easily, like Microballoons, should not be used below the water line as they can attract moisture. The fillers rated for use below the water line, like colloidal silica, are hard to sand. Apply the epoxy filler mixture as smoothly as possible and then sand smooth. If you get down to bare cast iron at any spot, immediately coat it with your converters and primers.
If you are a die-hard racer, you probably already know all about fairing your keel for better performance. There are companies that provide full-scale templates of the cross sectional shape of a boat’s keel. These templates are used, with lots of epoxy/filler, to optimize the shape of your keel and rudder. Great for extracting the last few ounces of performance from your boat but a major undertaking.
At this stage of the project, some people choose to apply a layer of fiberglass cloth and epoxy. This forms an additional protective layer against something penetrating the coatings and providing a start for rust.
Once you are satisfied with the surface, roll on a couple of coats of barrier coat epoxy. Make sure the barrier coat is rated for use under water and in conjunction with bottom paint. If you leave the boat in salt water or brackish water, give the newly finished keel a coating of bottom paint.
If you are using soft or ablative bottom paint, give the bottom three coats. Make the first coat a different color than the rest. When the first color starts appearing, its time to reapply bottom paint.
This procedure is fine for keels that are really rusty. If you just have small patches of rust and don’t have the time to do the full keel, at least take care of those spots. If won’t be as permanent as the full-blown keel job but it will keep the rust at bay until you can do the whole thing.
Well, there it is, Keel Finishing 101. Not the easiest job in the world but worthwhile if you want to keep your boat in top shape. If you decide to sell the boat, it will go a long way in keeping up the resale value. Nothing looks worse than a keel dripping rust and full of rust pockmarks.
An example of a keel let go too long!
A close-up of the rusty keel, a lot of work here!
All blocked up to refinish the cast iron centerboard.
A close up of the centerboard with several coats of metal primer on it. Ready for fairing!
A fully sanded, faired, fiberglassed and bottom painted cast iron keel.
Same keel, ready to go in the water.