Today at the Chandlery I found out a little secret... the shops that sell Interlux epoxy and paint products don't recommend using POR-15 because they don't carry it, and they make money by promoting the Interlux primers instead. Unfortunately Interlux doesn't manufacture a product like POR-15, so if you take the advice of the Interlux dealer you might be dealing with rust every couple of years on an iron keel. Interlux makes great products and I recommend them below, but for an iron keel you should start with POR-15 first, and then move on to the Interlux.
I refinished my keel in last spring and it's still clean as a whistle with just a smidgin of oxidation around the hull keel joint, which I didn't have time to finish properly. I'm really happy with the results so far and will probably carry on and deal with the hull keel joint this month or wait till next year.
(Update: as of spring 2014 the keel is still in great shape, and the hull-keel joint looks good, too. That's seven years and counting. Only the very bottom of the keel, which I couldn't reach, has some rust-scale on it. A project I hope to tackle in stages by raising the adjustable cradle pads and putting some 2x4's under the keel when I haul the boat out in late fall.)
This is a pretty big job, but it's doable. As a one-man job it took me about two weeks of steady work on evenings and weekends, without having much prior experience. Given the motivation, perserverence and elbow grease, it can be done by an amateur. There's always the alternative of hiring a pro to drop the keel, sandblast it, and professionally refinish it, and if you have a grand or so to spare, you might consider it. If that's not an option and you can get an extra set of hands to help, I wouldn't hesitate to tackle it. Here's what I recommend:
1. Get the old paint and rust off.Use a grinder for this. Scrapers don't work, drills don't work. Grinders work good. I used a very aggressive sanding disk (designed for metal) with a backing plate, and also the meanest looking wire brush attachment I could find in Canadian Tire. My first attempt was using scrapers and they did almost nothing but remove some surface rust. The sanding disks worked well but I went through a half dozen or so rather quickly. The wire brush was pretty good as well, and by alternative between sanding disks and the wire brush, the keel cleaned right up.
It's very important to wear proper protection for this step including a respirator and protective glasses. The dust from sanding and grinding the bottom of a boat is toxic, nasty stuff and you do not want to breathe it in or get it in your eyes.
It's not essential to get down to bright metal. You're not setting your table with it. The bits of original primer that stick on the keel won't hurt. With a grinder you'll quickly discover all the rust pockets and make short work of them.
Some owners with heavy rust scale have reported that an air hammer works best. It depends how bad your keel has deteriorated but if you the rust is coming off in heavy flakes, renting an air hammer might be your best option.
2. Metal-ReadyAlso known as POR 15 Metal Prep, you can get this at Canadian Tire now. This is a recommended prep before POR-15. It contains phosphoric acid. You can get other brands but this is the one I got from the truck shop that sold POR-15. Spray it on, and any remaining rust or oxidation that has happened since your last grinding session will magically convert into a zinc phosphate. You're supposed to wash this off with water. I was very reluctant to wipe my bare metal keel down with river water, but I did, and it was fine. Metal-Ready is a water based solution itself. It didn't start to rust like I feared, but instead rinsed most of the residue away and left a good surface for painting. This stuff works amazingly well to obliterate rust, and the zinc phosphate film left behind is apparently a good prep for POR-15 to bond with and helps prevent future corrosion.
3. POR-15You can now get POR-15 at Canadian Tire. POR-15 is hands down the single best rust-preventative coating on the market today. It forms a chemical bond that converts rust into a rock hard compound and locks out corrosion. Unlike other coatings it actually cures and becomes harder in the presence of moister, so it's ideal for underwater applications like keels. I did three coats with about 8 hours between coats. It dries ultra-hard so you need to put the next coat on while the previous is still a bit soft.
4. Interprotect 2000This is the famous epoxy bottom coat. I did not put this over the fiberglass hull because my boat shows no signs of osmosis. On the recommendation of a local yacht repairman, I did five coats over the keel, very lightly sanding the POR-15 first with a fine grit to provide a bit of tooth. I had no problem painting Interprotect 2000 over the POR-15.
5. Fairing compound.This is getting to the last stage. Fair the keel with a fairing compound and sand the compound smooth once dried. Be very careful when sanding not to sand away your bottom coats. I applied fairing compound only to a few spots that had obvious pitting, and then gently hand-sanded those spots smooth afterwards, being careful to stay away from the epoxy bottom coat. The fairing compound was blue and the interprotect was gray so it was easy to tell the difference. How much time you spend on this depends on whether you want a racing bottom or just a respectable cruising keel. Mine falls in the "respectable" category but I'm likely to do some more fairing work with time and conditions permit. Because time was running low, I faired up the hull keel joint without caulking. I recommend treating the hull keel joint differently however. I'm planning to dremel out the keel joint and caulk it. Filling it with fairing compound was a timesaving measure to be ready in time for last year's launch.
6. Hull Keel JointUpdate: I have written about fixing the hull-keel joint here:
The basic process is to clean out the joint with a Dremel tool, dab some Metal-Ready onto the exposed keel edge, paint with POR-15 using a small brush, over-coat with Interprotect 2000, fill in the gap with Sikaflex 291 caulking, and then paint over the whole mess with VC-17 antifouling.