As you can see there is a lot of masking tape on this holding the tares together. If left on the tape will cause glue stains to blead through to the front of the poster and break down the paper. You can use solvents to remove the tape but have to be careful to not use something that effects the ink. I oppted for the less smelly heat option. I used a hair dryer to loosen up the tape's glue and then lifted up a corner with an exato knife. Gradually I peeled back the tape bit by bit with tweezers and the hair dryer still on hot to keep loosening the newly exposed tape/paper joint. It took about 90min to remove all the tape.

* this is not the poster in my bath tub shown next but its in about the same condition.