Hi,
To separate Topsheet from sponge..
I've tried the following ONLY for Long Pips, and it has worked for me almost every single time, and without causing any damage to either the topsheet or the sponge ..
What you need to do: use a large thin towel or handkerchief, put the top sheet side facing down on top of the towel/hanky, sponge facing up, cover the sponge with the towe .. It's better if the cloth is a bit wet, place a hot steaming iron on one of the corners, any corner will do for minutes until the cloth is dry. Open the cloth and check to see if the sponge and top sheet is slightly separate, try to peel at that hot corner. If it didn't, continue. If it did, even if it is slightly, you are in luck. Be patient, try to steam and peel little by little. You'll known when to iron steam the whole sheet to peel the whole thing off. At times, I'll place the hot iron on the entire sponge, not just the corners..
For more stubborn glued sheets, I will use the above, in combination with acetone..