We have used the company, Fossil, which is high pressure laminate signs (come in many thicknesses). This is a permanent solution. However, depending on the size of your sign, they can be quite affordable if you are doing 1/8" thickness. A 6"x6" sign is approx. $20 which is what we use for individual animal signs in our zoo. There are other high pressure laminate companies but I have had great success with Fossil.
I am now just noticing that some of the Fossil signs need to be replaced after 8 years in the beating sun. They haven't faded but the edges are starting to look worn.
For temporary signs, if they are small we print them on an in-house printer and laminate with 10 mil UV lamination film. It holds up approximately 6 months depending on weather.
We have also done sintra with vinyl wrap and had similar success. You may want to check with the printer to see if there are UV inks they could use.
One other thing I have tried is printing on Tyvek. Tyvek banners are more expensive than Vinyl banners but are made for outdoor use. They are recyclable and better for the environment than vinyl. They are a little tricky to figure out how to mount to a backboard. I can't recall what I did. I think I wrapped a backboard with them.
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Tina Keegan
Exhibits Director
Palo Alto Junior Museum and Zoo
Palo Alto CA