When I use SceneColor in a translucent material separate translucency does not work. I can’t find any information on whether this is a limitation or not, except [another question][4] about SceneColor and separate translucency in particles. That question is similar to this one, but issue UE-15433 states that SceneColor breaks Separate Translucency for particles when SceneColor is used. This is probably the same issue, but not with particles.
If I don’t use SceneColor for the transparency the material is opaque and seems to work (not affected by post processing).
Don’t know if you’ve found a solution to your situation, but I had to face a similar problem to your recently while also doing a water shader and found that the problem is in that situation is not the separate translucency. If the normals are not showing as they should in what you’re trying to do (to have something like the the mock up), the problem is with translucency itself that currently doesn’t take normals and roughness into account. Notice that it doesn’t cast shadows either. Translucency doesn’t have the lighting calculated as opaque and masked materials and in order to come close to what you want if have to change the lighting model of your material.
In the details panel of your material:
Go to the Translucency tab.
In Lighting Mode select “Surface TranslucencyVolume”, like bellow.
Now your shader should behave as you wanted!
It’s important to note, however that it greatly augments your shader instruction and texture samples, so must be used with caution in order to create optimized results.
