To use ZEP or not use ZEP, is that the question? Not sure there's a one-size-fits-all answer. I do understand that some folks want decades old trailers to shine like new... that's not me, I kinda like the patina that happens with age. I see it on my face
I think what final application, whether floor polish or carnuba wax or ?? has more to do with time/effort/money. You will not get all three cheaply
unless you use something like ZEP.
I look at ZEP as being a last resort before the decision to paint
and prep can be expensive, ZEP not so much.
But ZEP or silicone
or even good wax, ALL needs to be completely and positively removed before paint
, or you end up with something that looks like this