Encasing Steel Keel in Fiberglass

[Stein]

The right treatment for a steel keel, or any other metal keel, would be to exchange it (along with the boat) with some all fibre reinforced design located on a cat or tri.
Sorry, can't help being a multihull enthusiast. (Read as: Fanatic).

I have no interesting experience with sealing cast iron keels, but plenty of experience with various fibres and resins in other contexts. Answering the initial question in this thread, I'd say no, don't cover the keel in any glass fiber laminate.

Epoxy is a good sealant, as it in basic form hardens as a chemical reaction rather than by evaporation, so it's non porous. (Poly urethane do so too, while polyesters evaporate large amounts of styrene, leaving lots of pores.) Epoxy also has better adhesive properties than most other resins. Note that different epoxy products can have totally different qualities. The basic epoxy is mostly made by three huge companies, (Ciba Geigy(the inventor), Shell and Dow) but they all have lots of very different varieties, and different brand companies mix in lots of other stuff like fillers, solvents, silica, etc, to change properties more, and reduce price... Water tightness and corrosion protection will vary much.

Adding fibres to the resin will make it much more resilient to mechanical abrasion (like the keel bouncing off a sand bar) and will make it able to carry bending and shear loads infinitely much better. If you need to build up large dents in the keel, or try to reduce cracking between hull and keel (it will be improved but not stopped), fibres are useful or necessary. The steel keel is of course way stiffer and harder than a thin layer of fibres, so you'd get no bonus apart from this local rigidity in soft / weak spots. The resin will not be more water tight with fibres in it. The total layer will be thicker, but seen in a microscope lots of tiny bubbles tend to create semi channels along the fibres. (Massaging the laminate with vacuum compression followed by autoclave compression will reduce this greatly, but this is hardly realistic to most...) So a laminate will tend to be somewhat more water penetrable than a pure resin layer.

Conclusion: Degrease, clean and dry the keel well, apply a good epoxy sealer (raw epoxy base would be ideal but hard to get and needs heating to cure) as instructed by the data sheet, preferably several layers with chemical bonding (next layer within a certain time.) Then apply the normal primer and anti fouling. By following instructions properly it should work well, for a few years. As mentioned, the final solution is removing all the metal.