Guys, 5200 and Sikaflex are darn good sealants. 5200 dries with a bit more gloss, but neither one of them are very "flexible" after a year. As far as removal, they are permanent. IF you are waterproofing things like stanchions, or cleats, do NOT over look good ol' Butyl tape. That crap will flex for years, if you have a good mechanical fastening, just squish some butyle tape in there from an RV place (Not underwater, but topside is ok), and you are good. It will never harden. Cleats and things that have a lot of stress on them will benefit, because it will move a little, 5200 is strong, but usually it will just pull the oxide off of the topside layer up a bit and leak in a while.
I have 5200 all over my 26 footer, which at 4 years of me having it, has needed some fittings re-bedded. I darn near damaged the fittings removing them. but when I did, I could see small channels where water had come in because of the 5200 not moving, and the surface it was affixed to moving a little. Try the Butyl.
As far as aluminium, you could go all out acid etching and such, but I imagine if you hit it with a flap wheel and then 5200, you would be fine. My 225 Yamaha took a nudge with a prybar to get it off of the pod after the bolts were pounded out, and the pod was nice and grey when I glued 'er on there.
5200 is tough stuff, I recently ran across a saying on the web somewhere, at a boat yard, two things get cursed at the most.
1) The Trav-L-Lift Operator
2) 3M 5200/Sikaflex.