One of my dirtiest memories from past rainy brevets is fighting not
just the water coming from above but the road spray coming from my
friends rear wheels. I am guilty of doing the same to my riding
partners, I do use full fenders on my flat black tank, but even with
them there is plenty of water flying up to the eyes and face of the
rider behind me so this time around I added a plastic mudflap to the
rear wheel, it ain't pretty but I think will work. Two holes in the
fender two on the mudflap and hold things together with whatever
fasteners I happen to have.

If you have fenders with mudflaps and I happen to ride with you I'll
buy the coffee...

BTW, the other day I posted here my rain jacket had lost it
waterproofness mojo, well I bought Granger's Wash-in Waterproofing (In
case you go to an REI store they have it by the shoe department.
http://www.rei.com/product/720434) and it seems the stuff did actually
restore my old jacket's superpowers. On a somewhat rainy 35 mile ride
last weekend the jacket performed well. The same brand also has a
spray-on I used for my backpack. Good stuff and it is less than $15.

Carlos