I was talking to a park worker the other day (i get on well with them) and they asked if id be scanning the fields, i ofcourse said now, around the edges but not the actual fields (at least not till the season is over then im all over it).
The reason he asked was becaue another detectorist in town had gone to another field and had dug holes and left them open all over the field...really trashed the place, its like one good detecting job always gets more, but one a$$hole and its right back to the beginning, sometimes it might be good to find the public places you can detect, even grass on the sidewalks and show people you have the equipement, talk to them if they show interest and get yourself known as both a clean hunter, sociable and respectfulm, willing to show your finds or a piece of paper with pictures of your finds on them.
It cant hurt to mention you always go 50/50 if you find something valuable and return what can be traced to an owner even if you havnt 'as yet' but show your not a 'backyard vulture' or 'dug, done and run' merchant.
If you state you alwayd show all the finds you get to the owners at the end of the day and where you found them and they are more then welcome to hunt with you if they wish, make it almost like your doing them the service and they may well get interested enough to give you permission and yes iof they turn out wanting to talk all day them give them that, the stuff isnt going to crawl out of the ground and run away, ABS, Always Be Social.