The simple way round any problems with wooden frames or boards is to brush apply a very thin layer of resin on as a sealing coat, then allow to cure and key up with sandpaper before going any further. Seals the wood and prepares it for further resin application.
Mix 550-600ml to take into account mixing pot waste and you should be fine.
Also bear in mind that GlassCast 3 will cure to a rock hard finish, so any minor surface imperfections can be wet sanded out and re-polished back to a gloss., so you don't always need to flood coat the whole surface each time if its just the odd minor defect to repair.