When in doubt, just follow the instructions in the order they are presented in the manual. I don't see any harm in varnishing the ribs right now, but it's also not urgent to get them protected. I'd get the other wood bits installed and then varnish all the wood at the same time. You're less likely to waste varnish that way, and will use fewer consumables, etc.

I've had to do very little powdercoat touchup, and then only because of my own clumsiness in dropping a wrench or something.