You might try reading...... and learning or applying comprehension along with it.
My first post was about letting professionals do it, since there is more than a bunch or artsy-fartsy people indulging in this. How can the OP or anyone control how lackadaisical a bunch of people will be with primers and paints outside ?? You can't.
It doesn't matter what the tech sheets have on them. If someone spills or ruins something while doing this, the OP might stand a chance of getting in trouble for organizing this outdoor paint excursion without following proper procedure.
Why other than trying to save money would a bunch of fellow artists, fine artists, mind you..... want to paint something and cut out the sign shop doing it professionally ?? To SAVE MONEY.... like I said, that's all.
Instead of saving money and getting in their own way and possibly creating a problem, they could muster up other ways to raise money instead of cutting out the professionals.
Inquire with Joe Diaz, who is relatively good at putting together wall dog meets.... how he gets by letting every town person in on painting and taking care of their own things at a wall dog meet. For insurance and probably any other liability, he can't either. Why do you think this person would be any different ?? Sure, it can happen, but are you doing it within the limits of the law..... or have you at least checked ??