The test items were disks that I made years ago when I first began working with homemade polymer clay. Because of their age, I knew they were completely dry. This is important because finishing hardened objects before they're completely dry will cause the finish to crack. I glued the disks to wooden skewers and painted them with simple designs in order to tell them apart. I finished one with a combination of polyurethane (undercoat) and acrylic (topcoat), and the other two with several coats of acrylic alone. The acrylic I used was Aleen's Spray Acrylic Sealer.
I stuck two of the disks into pots and placed one in the shower and one on my fire escape. I gave the third one to a friend to stick in his yard. They stayed in their respective locations for several weeks (the ones in the shower and on the fire escape are still there).
Test Case 1
Finish: Polyurethane undercoat wit acrylic topcoat. Location: Shower.
Test Case 2
Finish: Acrylic. Location: Fire escape.
Test Case 3
Finish: Acrylic. Location: Yard (not pictured; shown here in potted plant).
Test Case 1, the poly-acrylic-shower disk, fared quite well. The front shows no signs of water infiltration.
The back however, shows mild water-wear along the top edge. It should be noted that this wear hasn't worsened over time, and may be due to the polyurethane undercoat not being fully dry before the acrylic top-coat was applied. It should also be noted that I applied the acrylic topcoat because the polyurethane was wearing at that edge.
Test Case 2, the acrylic-fire escape disk, shows mild cracking of the paint beneath the finish, probably due to it not being fully dry when I applied the acrylic. As in the case of Disk 1, the cracking occurred early and hasn't worsened.
Test Case 3, the acrylic-yard disk, fared least well, with severe cracking all over the front and back of it. This is probably due to its having been placed in a yard and thus subjected to more severe weather conditions than the other two disks.
Several coats of acrylic finish will enable fully hardened and dried (if they're painted) items made with homemade polymer clay to survive in moisture-rich environments for some time. Those placed outdoors will fare best in semi-protected areas such as fire escapes, patios, decks, etc.
Until next time --