I would spell it unless and until the other person signs PLASTIC back to me. Then, I would use that sign during our conversation.
I completely agree!
As others have said, one of the issues with "Plastic" in English is that it means multiple different things - "Plastic" is the name of a chemical compound, however depending on various factors it can be hard, soft, thick, thin, brittle, flexible, clear, translucent (semi-see thru), etc. For this reason it makes it difficult - if not impossible to have a single sign that really "works" for all the things we lump into the word "plastic".
The only thing that I feel strongly about is that the "Glass" sign used on Signing Savvy is NOT something that should be used as it's universally recognized as "glass" and would be constantly confused!
Actually IMHO -
Native ASLers should really consider emailing Signing Savvy and telling them that the sign they are using (GLASS) is NOT representative of how PLASTIC is signed throughout MUCH of the ASL community (in Canada & the USA) and that they should really consider changing it to being fingerspelled - or at minimum mentioning that the sign they give is also used for GLASS and is a region sign which will not be universally or even commonly understood to mean "PLASTIC" amongst signers.
I'd rather fingerspell "PLASTIC" and know for sure that my meaning was absolutely clear, than mess around with what the various signs (in this case since it's SO diverse even within one region) and know that there is at least a 50/50 chance that the other person will NOT interpret the sign I use for PLASTIC to
mean plastic