When I bought Billy Ruff'n (Van de Stadt Samoa 47 built professionally in steel) the first owner said the secret to keeping a steel boat Bristol is to "make her think she's not in the water". To do that you must start with a great
paint system and then the owner has to love maintenance, keep a large inventory of Dremel tool bits handy, and maintain a well stocked
paint locker. Repairs I made 4-5 years ago are still good, but it takes time and patience.
To give our prospective steel boat owner an idea of what's involved, lets repair a simple blister:
1/ grind it back to bright metal -- you have to find the edges of the rust spot and don't stop until you've found a clean
line between well adhered
paint and bright metal.
2/ Ospho it to 'convert' any rust that might remain in the tiny spots where the Dremel tool won't fit. (I never really understood what was going on with Oshpo, but I assume it strips off the O2 off the rust molecule and replaces it w/ something inert. Oxide becomes a phosphate?? Folks wiser than I might comment on this?)
3. Grind some more -- Dremel fine point diamond tools work well in the later stages of rust removal from tiny pits in the metal. When you've run out of patience you can stop.
(A note of caution when grinding rust spots -- have a shop vac handy to suck up the grinding residue. I didn't do this one day on a mooring and ended up with small rust stains down wind from the repair. The rust stains will come out with phosphoric acid.)
4. Apply a thin coat of etching primer (Interlux makes the one I use). Let it dry for at least an hour but not more than 24.
5. Apply one or two coats of
epoxy primer (Interlux primer is a two part that is then thinned).
6. Fair the repair spot with a two part marine filler, sanding between applications of filler. Depending on the nature of the repair it may take two or three attempts with the filler and sand paper to get it right.
7. Apply another two coats of
epoxy primer.
8. Apply two coats of finish
paint. Wet sand the patch spot with progressively finer grits to blend the edges of the patch.
9. Apply what ever you use to protect the
paint (polymer sealants or wax)
....et voila, a perfect patch. Note that it's only taken 5-6 days to get the various layers to dry. As I said, you have to love maintenance and a degree in chemistry wouldn't hurt.
The owner (or his boat yard, if the owner has big bucks) can deal easily with the exterior rust on hull and deck using something like the process outlined above. The interior is another matter. Here you really have to ensure that your boats designer and the builder knew what they are doing. It's said that most steel boats rust from the inside out, and I think it's probably true. Even in the best designed and constructed boat there are places inside the hull where the
paint just doesn't want to go -- for example, try getting a
paint spray gun pointed up at the underside of a limber hole deep in the hull. It's places like this that rust first. Welds are another potential trouble spot. Then there's the issue of how the hull is insulated and how the insulation gets in the way of water finding it's way to the bilge. There is little you can do to repair rust that may get going behind the permanently installed bulkheads (without ripping them out, that is). Doing a proper job of rust repair in places that are difficult to access is nearly impossible, so here is where you have to be confident that the builder and his
paint crew did a first rate job -- and even if they did, eventually nature will take its course and the boat begins to bleed. (A point worth considering: interior of the hull should be white so you can see where it's beginning to rust). If you find a boat with dark
paint in the bilges, you are probably looking at someone trying to hide something.)
I'll second everything said above about getting a surveyor who knows steel boats, but I'd also carefully check out the reputations of the designer and builder before buying a used steel boat. See if you can find other boats built in the same yard around the same time and talk with their owners. Make sure the surveyor gets out a bright light and small mirrors and really pokes around in the interior of the boat.
As for the strength of steel -- with a moment of inattention during the summer of 2006 Billy Ruff'n and I hit a rock at 6 knots. No leaks. We sailed it another 700 miles to a good yard where the boat was hauled. Damage to the hull was limited to loss of
paint and fairing compound on the leading edge of the keel. It was easily repaired for about $2800. (We also put some hair
line cracks on the leading edges of all the spreaders near where they join the mast). I am convinced that if we'd been in a lightly built fibreglass boat, our sailing season would have been over and the boat may have sunk.
Nothing stands up to rocks and careless owners like steel.