I used the marine grade Sikaflex when I replace some Stainless skin fittings 2 years ago, worked well no problems.

Sure, preparation, through hull fittings and cut corners.
I removed all plastics, prepare the hole, fine sanding, clean, put SS through fittings, SS vales and Sica from both sides. Next to it blocked 1.5 inch hole , ss plates from both sides and 4 screws, Sica - the same way.
If the hole is just a bit bigger or rough edges, don't try to cover with Sica only,
unless you have dingy on local pond.

Thanks for the advice guys, helpful as always.
Now, would you suggest removal of the through hull fitting and re-sikaflexing at periodic intervals or once in and sealed are they good for (was going to say forever - that might be pushing it) many many years?