I did it my way
 

I have extremely clean drains, and no water on the floor. Even so when it rained the interior became generally damp, so much so the seat belts would be damp. I noticed that the roof immediately absorbed water when it rained, so I sprayed it with a can and a half of silicone spray. Now the interior stays dry even after a Texas downpour. The silicone did run over the paint after the firs rain, but the car was waxed and it cleaned right up. Now the water beads up and rolls off the roof. :)

Someone will correct me if I'm wrong - but as I understand it, these roofs are supposed to absorb water on the top layer - it's the inner layer that channels the water away? So while it is good to treat your top, it's not supposed to bead up and drip off. Might be a sign of something wrong with the inner layer?

I would not use silicone spray.... Use raggtop or 303

Isn't it technically

I did it myyyyyyyyyyyyyyy way?

That being said - Porsche dealerships use silicon spray on the tops when they service the roof.

Yes, Porsche brand 'Verdeckpfleger', Convertible Top Waterproofer p/n 000.044.000.91

Downtown Porsche in Toronto:
p/n 000-043-204-40 Silicon Spray .2L