Concerning the sealing, I diluted the Gesso with about 20% tap water in order to get a smooth finish. Undiluted Gesso causes a grainy finish, which you probably don't want.
Putting on about 4 to 5 thin layers with fine sanding (Grade 400) in between each layer will suffice.
Where needed I also used light weight filler to smoothen/straighten the surface; before sealing that is.
"With great power comes great responsibility"