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I think you could use this informally, with friends/as a 'home sign'. Or if you were doing a speech/presentation where you would be repeating the word a lot you could fingerspell it once and signal that you are going to use that sign for platinum. For any new people you meet though they probably wouldn't immediately guess what you meant if you used 'P' the same as gold/silver, so then you may have to switch to fingerspelling, just for clarity. I think in most casual/informal situations and especially with friends it can be helpful to develop signs for a word you might use often, or that is hard to spell Just an opinion/my thoughts from my experience though, not a rule for signing in general.