Decal problem

[fixeighted]

I have a job where I need to 'clean' a transparent box and at first thought decals would do an amazing job of placing the splashes, which it does of course.
However it sadly came to nothing, as I cannot find a way to make them disappear over time without still being seen when looking from the other side of the
box walls.
Is there any workaround that can be made to work maybe?
Currently I am having to render multiple complex sequences and fade them in post, which is really frustrating and prone to error.

In the images you can see that the dirt disappears fully when viewed from one side, but from the reverse there is still the remains, basically the mask used, though it is the same
if I set the image alpha.
I also tried with a blend material but that's worse. I also tried animating the mask opacity but although the mask animates, when used in the decal the change is abrupt, so only
fully 'on' then fully 'off' on the last keyframe.

[hermit.crab]

How are you making the Decal disappear? Could you send us the scene? I tried making the Decal's shader use a diffuse and an opacity map for the image itself and then use a VRayColor as the Decal's mask to fade it out. There isn't a difference between the front and backside when done like this.

[fixeighted]

Thanks; yeah that is one way I don't think I tried, though I may have done and discarded it, because whilst it does it, sort of, you will notice that whilst the 'correct' directly viewed side fades out nicely, when viewed from the other side the fade is very abrupt,
so at rgb 1/1/1 it is still very prominent and then blinks out, which just looks wrong

[fixeighted]

oops forgot to attach

[fixeighted]

Aah, yes I do think that works. Sorry, I now note that you specifically used a map for opacity rather than what I did, which was to rely on the png alpha, which obviously is the root cause.
So unless I manage to break it you may have just saved me a ton of work