Astouffer512- Noting that 1) "weekend Detailer" or Pro, it ougtha be done right
and 2) I`d decontaminate the vehicle with ValuGard`s "ABC" instead (which would probably eliminate, or at least greatly reduce, that "marring from the decontamination step" concern)...
No, I don`t think you should do the prewash that way.
I don`t see the rationale behind prewashing with a degreaser. Unless the car is, well...greasy...I wouldn`t do that; degreasers emulsify grease and I doubt that`s your challenge. It sounds to me like you`d just like to get the car as clean as possible before you clay.
I like P21s TAW (have used many of their 5L jugs), an OPC too, but both are seriously lacking in lubricity/encapsulation and I`d never wash a vehicle with them unless I had a very good, specific, reason to do so.
Unless washing with the ValuGard "A" (to neutralize any acidic contamination via its alkaline nature, more than to just do its "super wash", which it will also do), I`d just wash it with a really strong regular shampoo mix. Even though you plan to do Correction, I`d try *really hard* to avoid marring the paint any more than it already is.
As long as you clay properly, with the right kind of clay/lube, you shouldn`t mar the paint much (I`m tempted to say "at all" but hey, stuff happens). Just remember that the clay oughta glide along on a film of lube, shearing off contamination that it bumps into (it does *NOT* "pull it out of the paint" the way some say). Don`t press so hard that it goes through that film of lube and contacts the paint directly. Be sure to inspect/knead/replace the clay *VERY* often as the moment it picks up a speck of abrasive contamination it becomes sandpaper. I tear the claybar into small pieces and pitch them (or save them for nasty-duty) as soon as they`re contaminated, which can also save your [bacon] if you drop the clay (drop clay = throw it away). That still leaves contamination in the pores/microfissures of the paint, hence the need to still use a Ferrous Contamination Remover (and, IMO, something to neutralize any acids that`re there but I understand that few bother with that).
Since (sorry to say...) I`m pretty sure you won`t bother with "ABC", I`d start by inspecting the vehicle and determining just what the primary decontamination challenge will be and go from there. Mechanical Decontamination (claying and the clay substitutes like the towels/sponges/etc.) is not a cure-all. If it has etching from bird bombs/bugs/acid rain, you should use something really alkaline to neutralize that stuff or it might (probably will) continue to eat into the paint; if it has lots of Rust Blooms/other signs of Ferrous Contamination then yeah you`ll need something to fix that. Different types of contamination call for different approaches.
Perhaps there`s some new SuperDuper Carwash Product that I`m unaware of that`ll handle all the various issues (need for extremely effective cleaning, ability to neutralized both acidic and alkaline contamination, plus ability to handle Ferrous Contamination). If so, I bet somebody here knows about it
EDIT: While I was keying in the long-winded stuff above, Bill D said it all nice and succinctly!
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