Since it’s stainless steel, wouldn’t painting them with something corrosive be even better? Like whatever bullshit they spread on the roads when it might snow where I live, that ensures there are no used cars cause they all rust out after a decade or two. If someome who etches their own circuit boards got their hands on those CTs there’d probably be actual holes in them. Ferric chloride can ravage stainless steels. Not something you’d want to let loose in the environment though I’d imagine.