[Jonno]

If I read right and you were painting the black part of the windscreen and the paint leaked onto the part which needs to be clear there is two ways.
1) Use a pointed end toothpick and scratch off the paint. (It smetimes wont work depending on the amount of paint on the windscreen).
2) Use 400grit sand paper to sand the paint off, then use 800grit, then 1200grit, higher if you think you would need to. Then use a polishing compound (future floor polish...ect) and polish the windscreen. I have never tried the second option but I've seen the results and it works.