I've been spilling gearbox oil all over the place today and went out with the intention of picking up some cat littery stuff. Completely forgot - straw it is again.
Go to a hire shop that does auto motive valeting.
You need a cemical called TFR (traffic Film remover)
In its neat form it litraly disolves ground in greese oil road dirt even underseal if left for an hour or so.
Its a mild acid so PPE is vital but I recon regardles how mutch crap you have to lift with TFR and a good brush a couple of bags of sharp sand to soak it up you will be done in a morning.
Once you have the thick up just a normal preasure washer and a wet vac will bring it up like new.
With an early start you could be cleaned dried and painted in a day.