When you do your next bottom job, try using an air grinder with a 60 grit
pad turning at the slowest RPM that you can get it to turn. It takes a
fairly powerful grinder to turn slowly and still produce enough power to
remove the barnacle base. A DA will not do it. I knocked the barnacles off
with a scraper and many of the bases came off but many also were firmly
attached and it was going to take hours to get them off with a scraper. I
tried my DA with an 80 grit pad and that was hopeless. A friend, that
frequently refinishes composite aircraft came by to give me advice on
repainting the topsides of the boat. He suggested the slow speed low
pressure method for removing the paint and remarked that it would work well
for the barnacle bases also...man!, was he ever right! I used a hard pad
rather than the soft foam pad that would normally be used with a good
quality cloth backed abrasive disk glued on. The barnacle bases just melted
off and the light pressure kept the abrasive from cutting into the paint
below. Here's another tip that he gave me.
Have both an 8" and a 7" pad and buy only 8" disks. The outer 1/2" of the
disk usually dulls first, so when the outer edge of the disk gets dull, cut
the disks down to fit the 7" pad.
I've done several bottom job over the years and no one ever sugguested this
method. Try it. It does take a little skill with the sander to get just
the right speed and pressure, but it's magic when you do.