Advice about Lightroom masks and "artefacts"
Looking again at your image (which I could not view while posting) it seems you may have painted your adjustment onto the darks, up to the sky - rather than muting down the sky with negative exposure as I had at first assumed. So some of my comments need to be reversed - in a case where you are raising exposure due to backlighting, you may prefer to use the better-exposed area (the sky) as your masking guide rather than the darker area.
It depends whether the underlying "problem" is darkening the light areas (with blown pixels causing an issue) or lightening the dark areas (with noise artefacts causing an issue). And depending how you do it, it may be worse to go across an edge, or to stop short, but the ideal always is of course, to coincide with the edge using the appropriate degree of softness.
But: there is a really convenient option since LR4 which makes that less critical: a local "highlights" adjustment applies principally to bright areas and has little (unwanted) effect on dark areas, even if your brush has strayed into those. And vice versa, if you are applying a "shadows" adjustment it is comparatively unimportant if your brush has strayed into light areas (on which "shadows" adjustment will have little or no effect).
If you use the single Exposure correction, however, then that has a clear effect on both dark and light areas - and you will therefore need to avoid any such imprecision.