Removing tough deck screws

[Odhin]

I have a bunch of rusting square drive #7 x 3" deck screws in the railings and ballisters of my deck. I removed some of them with my impact driver, but many of them won't come out. I've tried to remove them with screw outs, but the burnishing bit won't penetrate. I've also tried many different drill bits (titanium, cobalt, carbide tipped) to tap them, but they work for about 3 seconds before they're too dull and don't work anymore. I was thinking of using a carbide burr to remove the heads, but I don't have a die grinder to use it with. I was able to use a reinforced cutting wheel in a dremel to create a slot, but they still wouldn't turn out.

I want to save the wood because I'm just replacing the screws with stainless steel ones.

Any ideas of what I can do to get the screws out?

 

[big-all]

you have pretty much covered the options
i am assuming you want to replace the screws and leave the structure un touched
if you want to separate the components then you could try a plug cutter but you may struggle to get one to cut that deep and you also have the problem off having to cut bigger plugs to fill the holes