Before going to all the trouble of dismantling the rudder, I would sand or scrape back the fibreglass around the tube and see what state the tube is in. If the boat is in the water have a tube of CT1 handy in case the leak gets worse. Also try pumping a load of grease into the tube and see if any comes out where the leak is. If it does the tube is perforated and will need replacing. You should also treat the rust on the mounting plate especially underneath and paint it with at least two coats of 2k epoxy primer (perfectly safe using a brush) and then two coats of bilge paint.
On my boat the tube is fine, just needed to replace the mild steel plate work which should have been stainless.
Hope this helps.