There are a few things you need to look into.
First a major important heads up....
The wet model (as in the reaction of the physics engine to weather) is NOT finished. As the knownissues file in the install folder states, there is only dry and damp. So wet is not an option (yet).
I assume you already converted the complete road surface to Real Road? This is also very important
.
I am not sure how this worked in rf1 (I only started this since rf2), but there is also a wet AI line you should set. Lines are different in the rain, so you need to tell the AI that.
Do the laptimes vary on track with wet and dry? Or is it totally no difference?