I think on some graffiti chemical is the only way - with a rinse off but If i had a big job on I would want heat as well.
We don't use a great deal of chemical but do remove a lot of graffiti with hot pressure washing.
Tensid have always been great products with great support but expensive as well.
Speak to Richard Turk at Tensid and see what he will do for larger quantities
Good luck
Rob