[Uhehner]

The lower grade stainless will leave rust stains on your gelcoat and will require polishing now and then. The better grades look like new for many years. West Marine usually carries the better grades at a price. Take a magnet with you when buying stainless (and brass). If it has any attraction at all it will probably leave rust stains. Same goes for stainless screws. Remember it's "Stain-LESS, not stain-PROOF".
As a note, WINK takes rust stains off gelcoat. Got it at ACE Hardware.
 I see no reason to bond thru-hulls that are above the water line, (comments please). 
You can paint plastic thru-hulls with Awlgrip on the outside and they look great and last for years. I did that on those that were near impossible to change. I also Alwgrip my antennas and they stay smooth and shine for years. No more glass splinters.
Capt. George