I have a sequence in which I have processed twice, once for the sky and surrounding landscape and one for a building, due to the buildings lights needing a different colour balance. I know I can create a mask in photoshop and make part of the image opaque and the rest transparent so I can layer them over one another in after effects and thus use the relevant parts of each, but cannot remember for the life of me how to do this! Can anyone help! Cheers, Will.

I figured this one out myself - create a mask in photoshop with whatever you want to be seen in black and the rest in white. Save selection as an alpha channel. Load into AE and put into your comp. Then click on the layer you want to make transparent (eg the one below the mask) and go to Layer - Track Matte - Alpha Matte. Done.

You can also lock clips / layers to a Parent so if you keyframe motion on the Parent clip, the others will follow.

Thanks Karl. What if the clip was shot using motion control. Can you recommend anything for that?

Also, I am doing multiple versions of the same sequence, for different parts of the image. Is there a way of doing this that doesnt involve having mutiple copies of the RAW files, eg just having different xmp data? 3 versions of one sequence means even more HD space taken up! Thanks, Will.

Locking to a parent should work no matter what as long as the motion is the same.

For the different versions you could just edit once and move those XMP files to a new folder, repeat until you have all the different versions you need. Then just copy back the XMP files you need at the time.

Thanks, will try that. Should save a huge amount of memory.