Recently I took on a relining job for a friend to help him get his posters back in shape after they got wet from a leak. After pulling off the linen and as I was picking off the old masa I discover old masking tape residue.
As you can easily see, the restorer just took the carrier off and left the residue on the poster. Why is this bad? Well water doesn't penetrate the acrylic residue very well or at all really. This creates a weak bond between the masa and the poster. In fact when I removed the masa, these areas just peeled right off with little effort.
So what can be done? Well it's tough but is relatively painless. First soften up the residue with a solvent and then scrape what you can off the poster. The overall problem is that by this point the pressure sensitive adhesive (PSA) has leached and somewhat bonded to the paper. All we are fighting for at this point is better adhesion. Here is what the area looks like after some softening and scraping:
I guess my lesson in this is, that you never really know what a restorer has actually done to your poster. It is a mad mad world out there... Best of luck!