I've left my Heliopan VND filter on one of my lenses for a couple of days, with a Marumi step-up ring. And now I cannot screw the Heliopan off the ring - only the latter will turn, in the lens' thread!

I was hoping for some better materials in both (Marumi is Made in Japan, while Heliopan - in Germany); never occurred to me they might stick like that. Arghh.... I'll apply some grease to all my step-up rings' threads, but please advise on some DIY methods of detaching the two :)

OK, problem solved. I just put the stuck filter/ring on a flat piece of foam material (the step-up ring facing down), pressed down firmly, and turned the Heliopan filter - it let go!

Before attaching it to any of the lens/step-up rings for a longer time again, I'm going to apply some of the natural "grease" from my own nostrils to their threads... Have heard somewhere it's the least corrosion-prone way - is it true?

be carefull with grease. It may collect dust and sand and make problems worse.

Solutions:
1) instead of using more force, just very, very gently hold the filter between two fingertips and start turning. It is often because of the pressure one puts on the filter, that the shape becomes slightly less round, resulting in blockage.
2) there are those special grips, to keep good hold on a filter and deviding pressure equily around the circle, when using more force.
3) Put the lens/filter etc. in a well closed locked plastic bag en keep the filter and ring under warm flowing water. Hopefully the step-up the ring will expand a bit faster (because of the increase in tempereture) than the filter and the thing comes loose.

I'm going to apply some of the natural "grease" from my own nostrils to their threads... Have heard somewhere it's the least corrosion-prone way - is it true?

I remember being given 'the secret' to loading the mags of the Eclair NPR by a 'famous' AC. Run your fingers down your nose in a pinching gesture, and yes - digging into the pit above the nostril arches, then running the finger down one side of the film guide, the thumb down the other.

Grosses some people out, but it works for me.