I'm glad you posted. All sorts of things happen and we feel that we just have to keep going and "cover up". You made a good call and most of us would have done very much the same, I'm sure. I ( even after many hundreds of hours of instructing) would doubt my capability of recognising what was on the windscreen. I have to confess, however, that I probably have something in common with your student (is this a gender thing or am I being unneccessarily wimpy ?) and may waited until the shower passed and the prop cleared my windscreen. What do others think ?