I've got a J/22
: (sailboat)with a hairline bottom centerline crack showing in the gelcoat.
: I assume this is a stress crack. To avoid blistering, TPI, the maker, uses
: vinyl ester in the first lamination after spraying the gelcoat in the
: mold. Then I assume they lay up the rest with cheaper polyester.

: Have you heard of any problems with the bond between the vinyl and
: polyesters?

: My intended repair will be an Interlux epoxy coat followed by bottom paint.
: Know of any reason I shouldn't use this approach?

Generally vinyl ester covers the middle ground between polyester and epoxy in terms of both price and performance. There should be no adhesion problem, but there are so many unknowable variables from the manufacturing process that any opinion at this point would be an educated guess. Certainly an epoxy repair would be superior to either of the other resins. But that leads to another issue.

If indeed it is a stress crack, epoxy paint won't hold your boat together. I'm not familiar with this craft, but I have seen some 'glass powerboats with semihollow keels, sort of U-shaped in cross section. This type of design of course causes a weak high-stress point, which is likely to deflect under load, and is extremely likely to crack. An acquaintance of mine had a 32' Bayliner with this problem, aggravated by a chance encounter with an uncharted rock in the San Juans. He was an epoxy formulator, so he pressure-injected a low-viscosity high-modulus epoxy into the crack (his own brew, but similar to Sika 35), and then filled the trough in the bilges with epoxy mortar. That was in '92, and last I heard, the boat still had the shiny side up.

So the structural problem should be diagnosed and repaired before all the sweat, time and expense of a repaint. It could just be cosmetic, or it could be signs of cancer. Maybe a second opinion by someone with 'glass hull experience is in order. After the repairs are either completed or vetoed, your choice of coatings sounds just fine. Keep us posted, OK? Good luck.