think this is my first post so will try and make it a good one lol.

make sure you lube it up well as if metal filings you could end up marking the paint. any pics of the problem.

personally i'd d othe following.

power wash down with jet wash
pre wash with a good foam gun and foam - leave to soak and run off the car
jet down again
refoam with a less foamy mixture
2 bucket wash with good lambswool mitt. make sure to rinse it out after each pass in this case
jet the car down
pat dry the car with a good mf drying cloth
inspect the paint surface to see what's left on there
apply plenty of detailer as lube start to clay the car a section at a time and keep checking the clay to see what it's picking up. keep moulding the clay to get a fresh surface to work with. pat dry as you go and keep inspecting. now that the colder weather's here i find it handy to have a pot with warm water in to help make the clay supple.

again after this i'd jet the car down
refoam - leave to soak and run off
jet down
rewash with 2 bucket method
pat dry again

you will most likely need to polish to remove micro marring left by claying and then of course re protect. sorry if you already know most of this. just my 2p worth on how i'd go about it.