Really, TDZ? Paaast, plaaaastic, faaaather, baaath are not only easy to imagine; they pop out of my mouth quite naturally. Say aaaah and you're haaaalfway there.
By the way, paste is payst. Aaaah is what the doctor asks you to say.
Paaaaassing on....
Can't his past simply be a bit dodgy?
I know this is stronger that taaarnished and chequered (as these two words don't necessarily mean he's done anything illegal, whereas dodgy does), but by talking about his past, you're already in effect saying that he's got good and bad in his character, the latter of which he's stepped away from (one hopes).