Natural consequences. Not point having this battle. You aren't with her all day and so you can only win it some of the time. Let her go out with jacket not fastened. What will happen? She get cold, yep. She not allowed out for recess? Yep. She get wet? Yep.
Natural consequence. If she wet, she stay wet. She 8 not 4 so live with consequence of decision.
I bet if you just stopped talking about it, and trying to persuade, she'll learn hard way and if she doesn't, only her who uncomfortable, wet and cold. More fool her.