Husband and I were talking about things that people, who know about a hobby, know not to do, to the point that they take for granted that people know not to do it, so they never talk about it, but people new to the hobby look around and don't see other people doing it, so they assume that they're the first person to think of it, instead of wondering if it's known but not done for a reason
The first thing in the fashion doll hobby I thought of like this was using Sharpie to do fine lines on repaints. When you're new, that seems like a much better solution than using paint on a brush; when you've been around for a while, you know that the reason it isn't done is because the Sharpie pigment gets into the vinyl and starts migrating away from where it was applied
Another is using acetone to remove paint on bodies. It works fine removing paint from vinyl heads because the plasticizers protect the plastic from the acetone; there are no plasticizers in hard body plastic, so the acetone immediately starts dissolving it
Polypropylene hair fibers. New people think the old people just don't like how lightweight and prone to tangling it is, so the new people think that when they deep condition and gel it so it feels and behaves better, then they've thought of something the older people just haven't. Nope! People who have more experience with dolls know that polypropylene hair fibers will basically self-destruct after 20 or 30 years, due to polypropylene's inherent property of having its polymer chains basically starting to unravel after any exposure to a subset of the wavelengths of UV light
I know there are more things like this, but I can't think of them at the moment, so feel free to add--with the explanations of why it's not a good idea. Just telling people "You shouldn't do that" without explaining why (especially when it's a measurable physical/chemical reason) can often lead to people yelling "Rules are made to be broken!" and doing it anyway. In the case of doll customizing, it's not an arbitrary rule, though. It's materials science
