having a problem just now that hopefully is just me being stupid.
I have a backplate on a plane, with a glass balustrade in front of it. When I have the glass material as ‘thin’ all is fine, as soon as I untick ‘thin’ then my glass goes milky white.
its fine in front of corona sky btw.
Fixed it,
seems ‘cast shadows’ being unticked in object properties of the backplate was screwing things up.
why would this be??
Because it is fake. 
Use the rayswitch method for the backplates: https://coronarenderer.freshdesk.com/support/solutions/articles/5000515605
already was using the rayswitch method.. but isnt it strange that cast shadows being unticked makes a difference?
things like this are very useful to know for people coming from vray to corona
Oh, you unticked “cast shadows” in object’s properties. Sorry, I thought it was in CoronaLightMtl’s properties.
Then take a look here: https://coronarenderer.freshdesk.com/support/solutions/articles/5000516723
thats it!
thanks, very useful info