They'd also have to paint without anyone seeing them paint. The fact that Vermeer stood out for being so secretive about his process says something here. Even a no-lens camera obscura would be pretty hard to hide given that it would require to be painting in a darkened room next to the person being painted.

But your question is an interesting one, for sure. Revealed secrets come in different flavors - fully known ones, but also "known unknown" types of secrets, like the exact "recipes" a painter might have used for their paints being a mystery. However, when it comes to "unknown unknowns" hiddeo secrets I think it's very hard to keep those when dealing with more than a handful of individuals.