Quoted from kciaccio:I put a playfield protector down the first month I got my game. Now I don't have to worry about dimples or wear.
I bought my second-hand alien from the seller that claimed that the PF protector was installed before the first ball and the playfield already shows major dimpling, just like in the pics above. So if this was true and the PF protector was there from the start, the PF protector might not protect that well from the dimpling regarding Alien playfield. I asked this from playfield-protectors.com and got this reply: "Regarding dimpling – protectors help but many and high impact airballs can have some effects. Depending on the „geometry“ of the machine, more or less airballs happen."