I've built in a super humid area. Super rainy. Had some mold growing on the wall. You can use lyme plaster, it's what i used, very durable plaster and no mold at all. After the cob dries you won't have many issues. Build a roof on top of construction to allow drying without having to cover the walls when it rains. Main issue is straw going bad. I actually had plants growing out of my wall during the wet winter season here from straw sprouting. Keep the air moving. You'll do great. Cob is very permissive.
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