Poster Paint is just an inexpensive form of Gouache, and most people will tell you it will not last long without some degradation. While I cannot refute this, the Alphonse Mucha replicate I did over 30 year ago shown here is still looking good! I was a huge fan of his work and did several reproductions many years ago. If you are ever in Prague, you must visit the Mucha Museum.
See a few more examples in My Arts & Crafts page.