When you're painting the bottom, just how "fair" is "fair enough?" Im getting ready to paint the bottom of my 1970 Ericson 23, but there is some rust on its cast-iron keel that is not sanding out easily. How smooth does the surface need to be before I use a filler and before I paint?
Tom Wood responds:
There is no question that a buffed racing bottom is faster than a traditional, rolled-on, and somewhat lumpy cruising paint job. But not many of us want the attendant prep and finish work required to get a perfectly fair bottom job, and a few brush marks, deep scratches, and roller stipple marks are only going to slow the boat a few tenths of a knot anyway. I've seen boats with bottoms cratered very much like the moon's surface that seem to sail along very well, especially when the breeze is up. I suspect that their light air performance suffers, though.
Cast iron keels present a more difficult challenge. As the fairing compound or other coating begins to fail, they will bubble up in large blisters of rust that sloughs off even fresh paint. These rust spots only get worse with age. So, once in a while it is necessary to use a very aggressive metal grinder (a metal grinding disc on a large angle grinder is not too much) to cut these pockets of rust back to bare iron, then immediately etch the bare metal and prime with a metal primer. Once this is done, fairing with epoxy putty will provide a smooth surface for the final coats of bottom paint. This treatment, if done properly, will last quite a long time before another pocket of rust pox breaks out in the same areabut it may break out somewhere else next year. So keep the grinder in the dockbox and at each bottom job make the compromise between the work involved and the fairness of the finished job.