Seeking advice on sealer for porch railing "cap."

Hoping you could recommend a sealer that will protect old railing cap on 110 yr old cottage on Maine coast. The porch railing cap is a 12″ wide pine plank that sits flat atop a short cedar-shake “wall” instead of the typical deck rail and across that area are 12 posts that hold up the porch roof and gables above.  It is a bad design because with a flat surface, the rain or snow/ice just sits there, sometimes for days.  It has been painted for many years, before I took over.  The issue is water:  rain and fog and the constant moisture level from nearby ocean. Also on the Maine coast, there are big temperature swings with snow and ice melting and re-freezing.  I have attempted to fix this by during long stretches of dry weather and low humidity (during September) to sand down the worst places , let the wood dry in the sun, and then repaint with a couple of coats using Ben Moore Aura exterior latex paint. But those efforts have failed. We get bubbling and cracking and it looks terrible.  I should replace these wood planks with new plastic Azek instead, but that is hugely expensive as it would require removing all the support posts and huge labor costs. So, I’m hoping to seal it instead. But, what procedure and product(s) is the question – and then what about the paint – do I paint before I seal the wood or do I apply a sealer on top of the painted surface? I assume I will need to again sand down the surface and let the wood dry out, but then what?  We want to keep the color of the cap to match the trim on the house, most of which is the same Ben Moore paint which holds up well because water is not sitting on it, it runs off and the wood dries out. UV exposure is the main issue with that.  Your advice on what to do and what products to use would be greatly appreciated.  I’m happy to pay you for some consultation, send photos if helpful.