Caulk adhesion at wood and gelcoat joint

[telekitr]

We have a wooden cap rail that goes over the bulwark and topsides joint. It is not removeable at this timeframe. We stripped and sanded the varnish off all the exposed surfaces of the cap rail and dug out the dried up caulk underneath the edge of the cap rail between the wood and the gelcoat. Our question is whether it provides better adhesion (and hopefully better moisture barrier) to a) put our caulk of choice, Sikaflex 291, directly on the sanded wood surface in the gap between it and the gelcoat or b) put 2 coats of penetrating varnish of choice, Epiphanes, cut 50/50 and 75/25 for penetration, before laying in the caulk? In either case, we would do a light sand of the Epiphanes before caulking. Does anyone have experience in the two methods? We are trying to avoid water ingress up underneath the varnish that causes blistering.

[Hirilondë]

I never ever caulk or bed to bare wood when I will be varnishing!! (exclamation marks for emphasis of how strongly I feel about this) This leaves you with a mechanical seam where the varnish meets the caulk. By varnishing first you have an over-lap at the joint, where the sealant over laps the varnish, which is much more impervious to moisture getting in.

This caulking will never be as good as removal and re-bedding, but I think you already knew that. Your choice of Sikaflex 291, I suggest 291-LOT as the tooling and clean up window is longer, I think is a good one for this application. The added adhesive property over a polysulfate will add to the longevity. Yet when it comes time to do it again, or re-bed the cap rail, it can be reefed out reasonably easily.