IMO the use of rent signs is a bit of an urban legend. Before investing any more time into cleaning them off, try cutting nag gluing them. A little research of the msds sheets pm these shows that they are usually polyethylene, not styrene, and good luck gluing that.
Also they are digital prints these days, not painted. Seems like that stuff runs when you don't want it to and sticks when you try to remove it.
I design signs for a living. The one product that consistently ruins my signs, unless they are clear coated, is Simple Green.
Another negative I have found using sign plastic is that it is pebbly in order to reduce glare.
Styrene is available in large sheets and thicknesses from places like TAP plastic.
As far as the EPA is involved, It ought to be considered that flushing brake fluid down the drain isn't so great. Brake shops dispose of it properly and charge for that as well.