Hi Keever, 

on strip builds, on the insides, after i have done the best i can with scrapers and sanding blocks, i have used phenolic micro-baloon paste to fill in gaps/defects.  it's easy to make up with your MAS (or whatever epoxy system you happen to use) and it creates a kind of marine grade bondo and is easy to sand.

i use a spreader to push it into large gaps that would otherwise be too large to fill during a normal wet-out weave-filling process.  the link on the clc site is here:  https://clcboats.com/shop/products/boat-building-supplies-epoxy-fiberglass-plywood/phenolic-micro-balloons.html

after pushing into the gaps, i then sand it back....it sands relatively easily and then i proceed with glassing the insides.

i hope i understood your problem and the advice if useful.  if you link a picture to your post, its easier to ensure we understand the problem you are having and can offer appropriate advice.

best

h