Getting a better edge on sky selection?

[DavePinMinn]

Windows, current version of Photoshop.

I want to replace a sky against some mountains.  Sky Replacement doesn't do a very good job because it doesn't provide a sharp edge where the sky and mountains come together.  Unfortunately, neither does the Select Sky.  There appear to be a lot of "partially selected" pixels.  If you've ever used either tool, you've probably noticed all the extraneous selection areas that make the replacement look less than perfect.

I've cleaned up the edge, so the selection edge is good.  But on both sides of the edge, I have junk that shows through.  What's the easiest way to get rid of all those partial selections so my mask is pure white and pure black?  I've tried using fill on the mask, but because the selection isn't precise that doesn't work.  How do I get rid of all the partial selection junk?

[PECourtejoie]

Hi, Did you try adjusting the mask, with levels, curves, etc?

[DavePinMinn]

Yes, but the problem is the partially selected pixels I think.  since some in the white area are dark, pushing the levels or curves doesn't make everything white.  That's why I'm looking for a way to get rid of the partial selections.

I have a "workaround", which is how I've always done the selection in the past - don't use EITHER of the tools and just do the whole sky selection with the magic wand or the quick selection tool.  But that's how we did this a decade ago when Photoshop was far less capable, and it seems rather unproductive to have to still do this using those old tools when there are TWO modern tools sitting there that should create a virtually perfect selection...

I also tried taking each side of the mask, putting it in quick mask mode, then doing a Threshold on it, and that SEEMS to work, though it seems rather kludgy when Photoshop contains both a select sky and a sky replacement tool... Is there a reason both those tools create such an imperfect selection?