If the signs are certain kinds of plastic the permanent marker can sometimes embed ink the plastic. If you soak a piece of the sign in isopropyl and it swells then you likely have something the ink will soak into.

Some markers are more permanent than others I guess (I came across some that weren’t really bothered by isopropyl or solvent).

For speed I’d suggest a stencil and some nasty spray paint (the type that isn’t easily removed).

Or a dry erase marker if that’s a billboard. Color over, erase.