You don't need lerp to gradually change the object's opacity. Just do this:
Showercover.opacity = clamp( Showercover.opacity + speed*sign*dt,0,100)
- sign is just a variable should hold a signed number 1 (1 or -1) to affect which way the opacity should change
- speed is to control how fast it fades (5 is slow-ish)
Oh, and clamp is there to make sure it stops at 0 or 100.