My unit when new to me had goop like you talk of. I fired it up, cranked the thermstat to high, and once warm gently scraped the thermoset adhesive off with a steel kitchen spatula that had only rounded edges. A surprising amount came off.
Once cool, a rub with a cotton dampened repeatedly with white gas (naptha) done outside while wearing rubber gloves took the rest away.
My release sheet is always between my heated platten and my print for matting. It is a waste release sheet from a cold mount overlay free from a framing shop.