You may find when you get them apart, and yes, they have to come apart, that the spring on the bottom has worn into the one above it. This is true for the whole spring pack. When they wear like this, it creates a step, which the lower spring needs to ride up when flexing. Over time the steps get bigger and the amount of spring travel gets smaller creating a very stiff ride.
If you find yours in this condition, round over the lips and smooth out the steps best you can. If the steps are deeper then you think they should be, buy new springs!
I used a hand held grinder with flapper wheels to clean my springs. The material is very hard so it will take a little work on your part to clean them.
I would suspect a fine grain sand or glass beads would clean them up OK. Very course sand may be harmful, but then again, mine were so hard, the flapper wheels hardly touched them. I tried drilling holes in the ends to install teflon buttons, but never found a drill bit good enough to make a mark in them. The spring shops use a very heavy duty punch for this.
