I never ask permission, or inquire about rules, at any city park I go to, anywhere. Why would you need to? I don't get it. Would you "ask" permission to walk on the sidewalk there? permission to birdwatch there? Why is detecting different? If someone has an issue, they're welcome to tell you. You've lost the battle even before you start, by thinking you need to ask..... as if there were something inherently wrong with you, or your hobby, that you had to ask. Heck, think of the image some desk-bound bureaucrat will have: geeks with shovels leaving a mess. What do you think he's going to say?
I've posted this example before, but will do so again: In my city, we have an oldtown park that was detected, at will, since the dawn of detectors. Then a newcomer attends our club. At show & tell he heard someone say "such & such coin was found at Central Park". The newcomer raises his hand and asked: "I thought it was illegal to metal detect there?". The rest of us just looked at him and say "since when?". Turns out he'd gone to city hall and asked! It had never even occured to us you needed to ask to detect, to begin with. That was 20 yrs. ago, and.... to this day...... I, or anyone can still go there, unbothered. If a gardener approaches us, it will be to ask for help in locating sprinkler heads.
The only exception I would make to my park's stance is obviously historic monument type places. But parks and schools are open game, unless told otherwise.
