How would you do it? Also there is a very thin layer of something on the sign that won't come off unless you use a sharp instrument an scrape lightly. Any ideas for how to get that off?

Try a heat gun. Latex should peel off when hot. Just be very careful not to burn the sign.

Don't have a hear gun but I'll pick one up tomorrow.

Has anyone tried to put bleach on a white part of a sign before? I was thinking about trying it. Has anyone had any success with anything else to remove a very stuck on later of dirt/grime?

I just used citrus strip paint remover and pieces of wood shims to remove the goo.

I don't think I would use paint stripper on a painted sign. I have had good results from a product call "goof off" takes the latex off.

There is a specific stripper they sell at either lowes or home depot for latex. Maybe test a spot and it wont attack the paint underneath.