Removing cured proseal is no big deal but can be tedious. I usually scrape as much as possible off with this tool then (when possible) use a 2" fine or very fine scotchbrite disc in a right angle die grinder to finish up. In fact, using a scotchbrite disc is a great way to remove large gobs a proseal in a hurry, but the downside is that it also gums up your discs pretty fast. Any residue left on the surface can be wiped away with MEK. The pictures are of a typical plastic scraper used in the aerospace industry for just such a task. I'm surprised our favorite tool vendors do not carry these inexpensive and handy little tools as they serve well in many tasks. However, a quite usable plastic/plexiglass scraper can be fabricated from an ordinary automotive style ice scraper. Do not be tempted to use a metal scraper, you will almost surely scratch the aluminum.

Using such a plastic/plexiglass scraper, you may have to resharpen the bevel frequently but using a bench sander, the process takes about 5 seconds.
Rick Galati RV-6A "Darla" 76 hours