If you write with permanent ink markers available in today's market onto plastic containers and other various materials that do not have a paper like quality to fully obsorb the ink, then the ink will eventually wear off. Who hasn't marked baking goods plastic containers or Holiday plastic boxes just to see that the writing from the permanent marker has worn off, and now you don't really know what it contains 6 months later and you have to rewrite the contents on it again. You also have to open everything and look inside to see what it is. If you use an acid based etching type of ink then it would soak into plastic and other formerly impervious materials and truely be permanent as intended.
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