Careful with that armourall. I know Roger from Bintel recommends it too. But one Dob that had that done was Wombat's at Snake Valley. Apparently the armourall worked allright at first but then went sticky pretty quickly. The base on that Dob had the worst sticktion I've ever seen. Maybe temperature extremes do funny things to that stuff.
I'm too scared to touch it after having seen Wombat's scope.
Stanley knife will work okay for the milk jug washers. Use a bench drill with a sharp bit through the plastic sandwiched between two blocks of scrap wood for nice and clean edges.