You just need more elbow action. To repair something like your break, I would use thin CA to put the pieces back in place. The I would have used 1.5 oz fiberglass cloth over it, wet sand it with 220. Then cover it with .75oz fiberglass sanding with 500 wet. Your just half way there. I like wet sanding, because it eliminates the dust, so that it can be accomplished indoors. The laminating epoxy thinned with MEK or urethane reducer will flow on, rather than goop on. Brush the area first with the epoxy mix, lay the fiberglass over it while wet, and brush it out with the damp brush. No cards or scraper to push the resin around.
Here's a picture of my latest repair after it was on fire. I bought it from a buddy who wrecked it, but couldn't repair it.
420 and some good tunes makes it more enjoyable


This is a SM 1/5th F16. He had a flame out at low altitude flying high alpha. The fins went through the bottom opening to the still hot thrust tube. It caught the grass on fire.

Big hole!

Started with CA what parts were salvageable. Then used fiberglass sheeting that I made on a glass table.

Lots of layers of Microballoons and heavy fiberglass. I sanded half away of what I applied

Last fiberglass layer was with .75 oz fiberglass. This is for a better finish and an easier weave for the primer to fill.

Finished. You can't tell. Still have to mount the fins.