Before you use lemon juice as an acne free treatment you have to wash and clean your face first. If you are applying the lemon juice to single pimples (as opposed to the entire face) then you have to wash the juice off thoroughly. The only reason for washing the juice off when it is applied to the entire face is that walking around with a sticky face all day is more than just a little bit inconvenient!
Why do you think the juice of a lemon is such a good way to treat and get rid of acne? Firstly, lemon juice has an antibacterial effect which helps to kill the precise bacteria that assemble in the pores of the skin and then go on to create acne.