Now you`re talkin`See...keep thinking about this stuff, REALLY thinking, and some good ideas will come to you. That`s how I`d look at it too.
I`d do the chemical decontamination first so the only stuff that has to be mechanically removed is the stuff that genuinely requires that. I want to do the least amount of mechanical decontamination possible...maybe none at all!
The catch there, for me, is I`m generally thinking of decon as part of a once per year "aggressive wash" prior to polishing (or heavier depending on the vehicle). I think back to the ironx marketing stuff which shows clay breaking off the embedded iron level with the paint and the chemical getting in to do the rest. If you leave chemical action to dissolve the whole lump, you need a lot more chemical (at best) if you keep dousing until there`s NO visible reaction or (at worst) you leave the deepest embedded particles in place.
Also, in my case, I always seem to pick up mostly non-ferrous fallout. I am somewhat inclined to nanoskin off the aerosoled tree sap, overspray and industrial crap off the paint first, since ironx won`t touch it anyways. The you`ve exposed the most embedded iron to the chemical. [I personally have a heck of a time with the trees at the family cottage misting sap - the wipers go "sssh sssh" on the glass after a week/weekend there and you can feel it on the paint. Traditionally I throw on an extra layer of LSP before going ]
Unless of course you have wave after wave of touchless decon chemicals - IPA for sap, prepsol for overspray, ironx for ferrous fallout..... or lube, nanoskin and polish....
[EDIT: point I forgot - and what if I have iron that`s be subsequently covered by sap and other non-ferrous stuff - then Iron-X never gets at it ]
Several approaches probably work just fine and it may depend a lot on just how much of what kind of contamination you have
[EDIT - yes, I`m overthinking this
