@nacho_simul I recorded video and added subtitles, I think it’s better than 1.000 words 
It’s simple scene made in UE4 Third Person Character Template. You can see Scene Capture 2D settings there and to TrueSkySequencerActor I only assigned that SC 2D actor (didn’t change parameters in Rain Mask Width or Rain Depth Extent). Hope it helps to understand my strange behavior of rain masking. Link to video: TS_Rain-Test_01.mp4 - Google Drive