I tend to permanently keep a UV filter on lenses, mostly for general protection rather than anything else, and the other day I added an ND filter to one. Both were Hoya filters, good quality, and they are designed to be stacked. However, when I came to remove the ND filter I found it impossible to remove just the ND filter as the two seemed locked together.
Well, not ‘seemed’, they were well and truly locked. Removing both the filters I spend some considerable time, on several occasions, trying to prise them apart. No luck at all, wouldn’t budge.
Online solutions, or suggestions, included the use of rubber bands to get more grip or tapping them on a hard surface as you would a jar whose ld cannot be removed. Tapping didn’t work and I had no rubber bands to hand.
What did work was putting the filters in the fridge for 20 seconds – I kept the door open. I took them out and they separated easily and normally, as if they had never been stuck in the first place. So there you go.