-You have someone else scrape them; and then grind them to finish the job. Otherwise sand/soda/shell blasting.
Some of the most important to remove barnies, like around your swim gear
and through-hulls, are mechanically wedged into place as well as cemented on. They will not come off with some magic sonic doo-dad. If someone believes that is true, then they probably also order charts
with parallel longitude lines.
Shell gets cemented on, in some cases, with better adhesive
strength than your gel-coat does to a secondary bond of the fiberglass layup
of your hull
In any case, scraping barnies is a tiny tiny little cleaning
chore. It just doesn't take that long. Scraper on a stick, wide razor scraper for the waterline to get off the scum, and a hefty banger hand scraper for the rudder
roots and other swim gear
. Just do it.