Alot of good advice here. I would insist on a stain-locking 100% acrylic primer and two coats of 100% acrylic finish. Oil primers work, but as you have seen, they get brittle with age, cannot expand and contract with the thermal changes, causing the whole system to pop.
One other thing: Do NOT caulk the horizontal laps. This is like taping a fish's gills shut. The house breathes through these. I have seen really good jobs bubble because the painter did this.
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