I'd put a sacrificial zinc on the keel. I had a boat with a cast iron keel, and tried all kinds of fillers and barriers, but nothing lasted. The keel needed repair every year until I installed one.

 

My previous two boats had iron keels. I found that no matter what I did, rust spots would appear. About every five years or so I would strip the keel and apply several coats of Interprotect 2000 and then anti-fouling paint. That would stop things for a year or two but eventually the rust spots would reappear, and when it became bad enough, it was time to do it all over again.

 

No need to worry about rust on a cast iron keel. OP didn't say what material his was...

There are cast iron boat parts that have been sitting in the ocean for hundreds of years. That keel is going to outlive your great, great, great grandchildren's grandchildren as long as it's in the water. The stainless steel bolts holding it in are where you need to worry about rust and galvanic corrosion.

Technically you don't even need to paint it. But getting it cleaned up, cover with 2-part epoxy, then bottom paint won't hurt and will help seal it and the join to the keel bolts, keeping them dry. Then after that, don't sand down the paint anymore to the metal.

After a decade or more water may have eventually snuck in somewhere, rust has formed, and thick piece of paint may start to chip. Just rip off the chip, feather the edges to the metal with a grinder, re-epoxy, and then bottom paint.

Do it right and you should never have to sand/grind and re-paint an entire keel ever again on this boat (but you'll probably have to do it on your next boat as I always see people in the yards doing it wrong).

 

I have used rust converter with a degree of success as a quick fix. Wire brush, spray with converter, when dry prime and then bottom paint. (VC 17 in my case - fresh water.)

 

IMHO the key to a long term fix for a steel keel is 4-5 coats of straight epoxy, applied to metal that has been burnished white. A primer or Interprotect wont do the job, you need a thickness of hard epoxy to keep all moisture away from the metal. I've done two steel keels like this both which served for years with no rust, other than where damage to the keel takes off the finish, in which case you just need to re-finish the damage area. You can fair and finish the epoxy after the last coat.

 

I've done a couple of iron keels using the following process (an old post which I re-post when the question arises yet again). Before & after pics of the latest one attached.


From the research I've done it seems to me the best approach is to sandblast the keel white, then immediately apply a base layer of epoxy. Any recommendations as to what to use for the base layer? Is something like WEST systems alright, or is a zinc enriched epoxy necessary?

Sandblasting is correct - any grinding method will not clean ALL the pitted areas fully and future failures will start there. Just use regular epoxy resin as a seal coat - nothing fancy, just get it on right after blasting and a solvent wipe. You can't even leave this step overnight. West is only more expensive - twice as much as the industrial no-name resin I use. Stone fabricators - counter tops etc. use a lot of epoxy - check around and you'll save $hundreds.

Moving on to a filler, again is WEST ok? and how many coats is recommended?

Again, West only costs more. Get a cement bag of industrial talc for about $20. It makes the most beautifully sanding filler you have ever experienced. Mix it to peanut butter consistency with epoxy and you probably won't even have any pinholes to fill after sanding.

Many people claim to use a notched squeegee or trowel to apply the epoxy, and then fill in between the created lines? is this necessary?

This makes it MUCH easier to get an even coat. Unless you're an experienced plasterer, using a plain trowel will give you a very uneven thickness. You do NOT want to sand through to metal, ever, or you will have to start over there. You want a reasonably thick finished coat - 1/8" min. is my preference - in order to ensure the metal stays buried.

Get an autobody longboard sander - image attached - they look kind of like an old smoothing plane, about 18" long and take pre-cut strips of sandpaper. This will help you fair up your keel - mine ended up looking like it had been templated.

I finished mine off with 3 coats of epoxy resin and 3 coats of Interprotect (there was still discussion as to the best sealer at that time). Now I'd just use Interprotect for all coats.