Great question — answering as the UphillTech team:
It seems to have to do with the process of sanding, adding sealers and layering the acrylic paint in a specific order. It is going to need more than 2-4 layers of sealer and acrylic paint to finish the ornament. In my process I added a white primer as a base, but could have added a sealer on the very first layer, and then added a primer. Numerous layers need to be sanded to give it an abrasion, so the acrylic as well as the sealer has the ability to stick. Otherwise it is going to be ripped off whether it is because one ornament is resting beside another in storage, or just because it's held in the hand. source
Full answer by Lyssagal: https://crafts.stackexchange.com/a/10755 (CC BY-SA 4.0).
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