Hi Doug,
De-zincified screws – there are many suggestions out there; in the end it boils down to how much effort you want to expend! I’ve tried most methods with mixed success. If the screws are powdery, then it’s extremely unlikely you’ll get the remains out. If there’s a bit of metal left you might be able to chew away at the surrounding wood so a Mole grip can be brought to bear on what remains of the screw head, but it’s likely the Mole grip will just snap off the head from the rest of the screw. And then your still left with a rather messy chewed up piece of surrounding wood. I’m told a bit of heat will free up recalcitrant screws, but in extremis you fisk a bonfire!
Another method (which I confess I have used extensively) is to to carefully remove as much of the old screw as possible without damaging the surrounding timber, carefully drill down a bit and then plug it with a suitably sized wooden dowel liberaly coated with epoxy/waterproof glue. Once cured, the plug can be smoothed back level with the surrounding timber. But then you have to put a brand new screw adjacent to the old screw to retain the strength. Bit laborious, but who knows that the plug is anything else but genuine?
Good luck!
Mike