Photoresist PCB - Traces always gets washed away with developer
There are at least two curing mechanics known to me - first is UV light and second is heat. You may notice that traces on left PCB is much more accurate - that's an appropriate result for an UV curing. Right PCB, however, is ugly. It looks like heat traveled from traces outward and cured away photoresist. Also, film is really important - semi-transparent film is bad and soaked up paper is trash. You wont get any thin traces with that.

I would recommend moving exposure lamp away from PCB and increasing exposure time proportionally. That would help to reduce heat

UV curable photoresist really helps in creation of tiny traces and double-sided PCBs, however, with such huge trace width, tonetransfer technique would be superior