i'm a big fan of fairy liquid on the inside of the visor, only lasts a few weeks at a time but great when on and cheap too, has to be the original green stuff mind, also works on glasses for the same time frame.
when i'm up the mountain on the quad with no helmet i would have major issues with the wind watering my eyes which i think is your problem, i don't wear glasses but a mate who emigrated a while back visited home and presented me with a pair of harley davidson goggles which you pull on over your head with an elastic band round them. goggle part has rubbery foam "gasket" type seal around the eyes. well they kicked about the house for a good while as i didn't know what to use them for, then one day the penny dropped and i tried them on the quad...awesome job.
no more, and i mean never watery eyes through heat or cold or even rain.... i am on that quad in all seasons and weather.
should be no problem with an open face helmet don't know about full face.
worth a try i think.