I want to create a milk glass effect for a door. But the milk glass should be only visible at a part of the door. I already got the a mask but I don’t know how to combine everything.
Please help a noob - thanks 
Shows us a pic of your material -> in most of the cases you use a lerp node to achieve what you want
This is what my material looks like at the moment.
I already experimented with Lerp 'n stuff, but I don’t exactly what it does.
Frosted translucency? I think you will need render-to-texture to make that work right.
Create a texture render surface where the door will be. Create a render-to-texture camera where the regular camera is.
Then use this as an input into a material that makes a milky translucent surface.
Apply lots of blur.
Maybe there’s some additional magic that lets you do this single-pass; I’d be super interested in learning about it if there was!