How to play 18+ games without friends seeing?

Ok so I made a second steam account on a different email so I could play 18+ games without my friends/family seeing, but when I switched back to my main account it would still show it in my library and the play time of one of the test games I used, a non 18+ game because I was paranoid for this exact reason. How do I make it so nothing from my alt bleeds into my main? Serious answers only please.