Varnish lifts from teak because oils or water give off vapor under the greenhouse effect of a clear coat in the sun. The vapor pressure can be very high in small spots.
Water getting to the wood on the inside of trim or underneath cap rails may wick towards the surface. All the prep in the world won't stop the process; eventually the coating will fail. Oils OTOH allow the wood to breath but they have their own problems.
If you want to paint teak some varnish first coats will make removing the paint later easier.