I used a spar varnish on my first paddles, and oil on a latter one. With use, the oil-finished blade did not prevent water absorption, and subsequently the wood laminates swelled. This did not result in any cracks or failure, but it created some concern. The varnished blades were scratched and also allowed water to get to the wood.
I wanted a more durable blade and also wanted some protection for the epoxy tip/paddle joint. Fiberglassing both surfaces of the blade added between 2-3 oz to the final paddle weight, and in my opinion the durability offsets the weight increase.
I have used 3.2 oz and 4 oz cloths with West epoxy system 105/207 and also MAS Low viscosity resin & slow hardener. I think 6 oz cloth would create a heavy blade so I have not used it. Undoubtedly it would work and if weight is not a great concern…go for it.
After the epoxy has dried, trim off the excess cloth, turn the paddle over and glass the other side. The second and third coats of epoxy can be applied to each side without allowing the epoxy to cure.