I got a big job like this and its a pain, we wfp them and it does leave the overlap dirty and sometimes causes runs as the water gets between the gap then drips after.
We also have to do the insides which is also complicated as soon as you open the louvre the water just runs !! and they take ages to dry.
Best method I have found is have someone on the inside to shut all the windows then wfp them and then get the guy to open them a tad to let em dry and stop the drips!!
I hate this job its a real pain !!! even thought of breaking the windows and let em get new ones on my insurance !!
Rob