Just a suggestion - carry on planking the hull as best you can to get a firm base.
Dont worry about it being perfectly flat and even - you just want a firm base.
When all is done and dry, sand over the whole hull until you have a smooth and even surface. Dont worry about any gaps - unless they are quite large. If they are large enough, just trim up some plank wood to fit in the hole and glue it in.
Then give the hull a final once over with sandpaper.
Now, put a very thin skim of car body filler over the whole hull - making sure to fill any holes. Once dry, sand down smooth with wet and dry (used wet).
Once that is done and dry, you can now skim over with a thin mat of fibreglass tissue and resin.
Sounds like a lot of work - but then, model boat building usually is!!!!
Hope this helps.........
Carl