UV protection coating - secret inscription

Hello everyone!
I am looking for a coating which would help me execute a concept ilustrated in the picture.

It is about application of an UV filter (or something else) over a stencil with text. Then put this piece of wood under the sun and capture the process of a darkening of a wood where the uv filter is not applied.
However, it is necessary that initially (before illumination) you cannot recognize where the filter is applied.
The result would be revealing of a “secret inscription” after some time.

The object with that use will be situated in interior.

For now I tried to apply some matte transparent stain (glaze) with UV protection over stencil, but i can easily see the difference between treated and non treated wood. (of course!). – treated is little bit darker and glossy (although is should be matte). And also after sun, the uv protection coating did not stop darkening at all.
So i tried again apply the same stain over stencil, then i treated whole wood plate with stain without UV protection. I can berely see where the UV is applied (good!), but the result after sun is again bad – the part with UV came to dark as well. :(
Maybe it is because bad stain with UV protection. but I tried two different kinds of them.
Now i am thinking about trying the Mr.Super Clear UV Cut Flat Spray.

I am open and glad for any ideas how to make it. Thanks a lot!