While lurking in another forum I belong to I came across this method of replacing a sky. Maybe you know about this but thought I would post anyway ....tip came from a Tom Bennet
image..drag a picture of sky on top. You should now have two images in your layers palette change blend mode to linear burn (at this point you are most likely saying "is she nuts this looks like cra oops terrible) now add graffi's layer mask and paint with black to remove the terrrible parts. If you don't have Graffi's mask here is a work around. With the background layer highlighted click on the adjustment layer button to make an adjustment layerdo nothing but click ok (the little half black and half white button) now highlight the sky layer and go to layer>group with previous, now back on the adjustment layer make sure the white box is highlighted, you will see a faint outline around it. paint with black till you get it the way you want. I also played with lowering the opacity on the brush is certain places. here is a before and after picture . also my sky is did not cover the whole area I wanted covered, so I just used the transform tools to make it larger. I did this quickly so is not the best, Hope this can be of help to someone
image..drag a picture of sky on top. You should now have two images in your layers palette change blend mode to linear burn (at this point you are most likely saying "is she nuts this looks like cra oops terrible) now add graffi's layer mask and paint with black to remove the terrrible parts. If you don't have Graffi's mask here is a work around. With the background layer highlighted click on the adjustment layer button to make an adjustment layerdo nothing but click ok (the little half black and half white button) now highlight the sky layer and go to layer>group with previous, now back on the adjustment layer make sure the white box is highlighted, you will see a faint outline around it. paint with black till you get it the way you want. I also played with lowering the opacity on the brush is certain places. here is a before and after picture . also my sky is did not cover the whole area I wanted covered, so I just used the transform tools to make it larger. I did this quickly so is not the best, Hope this can be of help to someone