Varnish lifting at joint in toe rail etc.

It sounds to me that the joint is too tight. I'd slice the joint a little wider and then fill it with a material that expands and contracts and then varnish over it.

Joint isn't too tight. The end grains not being clean won't hold the adhesives used to bind them and their biscuits. Much of that is brought by moisture, very often introduced from beneath and through rub rail fasteners. Widening the seam allows for effective "cleaning" of the end grains exposed. The biscuits are still present. Epoxy will do its job. Movement is inevitable because, well, boat and UV. Yes, you could use a G-Flex type adhesive that allows movement, but by cleaning the grains I'll get good adhesive with the West System which I designed to bond to the wood well. By eliminating the moisture sources, much of the hyper expansion issues will be eliminated. At that point the joint will behave just like the rest of the cap rail. If there were no biscuits or butterflies or whatever techniques to additionally mechanically bind the joint, then I'd favor the flexible sealant.

cleanslate, 1/4' x 1/4' is width and depth. You do not need to go all the way through. The penetrating epoxy sealer is very thin.
Do make sure your cut is centered over the joint.

Not if you can eliminate the introduction of moisture to the situation.

You need to get to clean wood. That's all. Otherwise the epoxy, even thinned, is trying to adhere to the mold/etc and damaged end grains.

Why so wide on the cut? This method breaks my heart! Imagine doing this to a Trumpy

Somethings wrong if you can't even get a season out of properly varnished teak toe rails? I'm not sure even these cut and epoxy solutions are going to help you. Who manufactured your boat? How old is it. You may have a deck problem that the toe rail joints are just reflecting. How is your toe rail attached to the deck? is the deck/hull connection still strong and complete? Is there a cleat right next to the joint? Does your hull or deck flex? Do you run 30 knots thru 6 footers. Just some of the things to consider before you "cut" Once you turn that wood to sawdust you ain't replacing it.

Do you have a marine carpenter at or near your marina who could take a look for you?

If the problem keeps recurring, there are but two culprits: considerable structural flex or moisture. Most often the issue is moisture, and often it comes from places not obvious to the eye.

How are you hauling the boat? I managed a 63' Ocean SF that was a 1987, owner owned it since 1988, the first couple of times the owner hauled it with slings, the gunnels would compress in almost 6" and always crack the varnish up there. After that, we put 6x6" long blocks under the rub rail in that area to extend the load and it stopped it from cracking the varnish.

Please stop and consider what you just said above in bold. Once the varnish is laid, there is no opportunity for those end grains to get wet. In order for them to get wet:

• There is some cracking created by hull movement, be it temps, sea, lifting, that creates a portal for water from the top
• There is an opportunity for water to enter from the rub rail either via a fastener or seam in the varnish (because the SS rails weren't removed when varnished)
• There is an opportunity for water to wick up from beneath via the deck surface, an exposed fastener, some other location for water to collect

You cannot have water transport itself through the varnish surface and impact the end grains. So.....how is the water/moisture getting into position to impact the wood in that location? That answer solves your problem. If you don't resolve it, stop the moisture, whatever fix you impart will fail.

Cleanslate, if you keep it NE, cover it this year - snow. ice, rain, sun all ever changing are killers on the best of wood joints. Oceans have a great fit and finish, and generally the teak I have seen is more golden so it my be less difficult to blend in a repair? Good luck. Nice boat.

It's possible the moisture was radiating up into the toe rail from the old rub rail leaking.

Go for it! Let us all see how it works out. I like the idea of a thinner cut.

Exactly!!

Expansion, contraction, age, and the biggest culprit, Moisture! If you are able to keep moisture from under your finish, you are good.
Sealing the end grain, and I mean saturating until it will not absorb any more, will prevent the moisture from wicking into the end grain.
One must cut the rail to expose the end grain.