The best way to get plastic frames super clean is to build a "wax moth hive". Just put a bunch of old frames in a hive body, and stack them just like a hive, and have a bottom and lid, and then just wait a few months. After the wax moths have cleaned the frames, just shake off the webbing and then touch it up with a pressure washer if you want (but not necessary).
Doesn't work in the dead of winter, but it will other times.
Don't worry about waxing the frame- it will be readily accepted as is. I've used this method for years now, and it's the only way to go. Trying to pressure wash an old frame of comb is a pain... can't ever get all the old cocoons out and you end up a mess.
And don't believe the old wives tale that having wax moths around can hurt one of your hives... that's just not true. They only go after a hive that's ALREADY weak from something else.