Here is the advice from Sea To Summit :
SOFT OR STICKY TENT FABRIC PU COATINGS. Here is the only possible treatment for this problem of old age softening of PU coatings. Carefully separate fabric layers if they have 'blocked' together. It may not be possible to do this without some coating damage. Set your domestic iron on cotton, non-steam. With a piece of clean photocopy paper (80gsm or heavier) always between the iron and the fabric, work carefully over all the fabric. Keep moving to avoid excessive heating. Ironing will harden the coating and drive out moisture. If parts of the coating are damaged or missing (hold the fabric up to the light to see this most easily) it is possible to smear Seamgrip® over these areas. If they are extensive this may not be practical. Whether you Seamgrip or not, finish by spraying the coated face of the fabric with a silicon or Teflon based water repellent. Your specialist outdoor store will have a suitable product. As you should always do, store the tent unrolled, coated sides out to minimise self contact.
http://www.seatosummit.com.au/techinfo.php?Area=tenttech&ArticleName=DIY%20Tent%20RepairsFranco