Gearbox

I don't use any sealant, but obviously the faces should be in good condition....Simply a smear of grease on the gasket.

.. I find that one face is in slightly better condition than the other. I use grease on that one and wellseal on the other. Onviously this means the gasket is unlikely to be reusable but they're cheap enough.

I use Castrol "Waterpump Grease" for these applications, because that's what my Dad did!  It's nice and thick and can also be used on the Clutch band to seal the case, and doesn't set like silicon.

If you ever need to replace the inner gasket RGM sell a neoprene replacement, which should be re-useable, see part number 040030N.