You might need to do the windows twice, as the lead oxidises and there can be a layer of whitish powder on the lead, which leaves spots on the glass once dry.
So you might well need to go over them once and then return a bit later.
Just check the first window you do, once you have finished to see whether you will need to do them again.
If there are metal bars behind the windows you will be able to apply a bit more pressure on the glass, but if not you might notice that the glass already bows inwards, a sign that you need to take great care.