MrSnuggles - Barnacles are incredibly difficult to remove - some harder than others. Having worked in a marina and lived on a boat in my early years, it was not uncommon to scrape the bottom of fiberglass boats and have part of the gelcoat come off with a barnacle directly exposing the glass fiber. They would attach to rudders (painted or not), props, prop shafts, and of course the hull. A long weekend in the water was sufficient to start barnacle growth, and they never fall off. Our typical procedure was scrape and then sand the residual portion of the barnacles off of the hull - often multiple times each boating season. That's why the anti-fouling paint (about 40% copper as I recall) was so important - at least you sand the paint off and not the gelcoat. Here's a decent link which gives you and idea of just how potent the 'glue' is.
Scientists finally unravel the mysteries of barnacle glue | Science! | Geek.com
Given their propensity to attach to smooth surfaces in our area (that may be dependent on the barnacle species and I'd defer to a marine biologist on that), it very much surprises me that there is no barnacle growth on the 'flat' surfaces on any of the pieces. Then again, we tend to get what's loosely called 'acorn' barnacles here and it seems the flaperon had 'goose' type growth. Attachment points look to be significantly different.