OK, here's the first question. Is it the water based version in which case getting thinners for it isn't difficult?
If not then in the past I have thinned the stuff with dichloromethane. I worked in a lab so that was easy for me. Dichloromethan is used in some paint strippers but distilling it at home is a lot more trouble than buying a new lot of liquid paper.
Since you don't seem to have anything to lose you could try whatever solvents you can get easily. Acetone, white spirit, methylated spirits or whatever.
Of course don't forget that solvents are often toxic and flammable.