Thermochromic paint.
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Previously:
https://www.homemadetools.net/forum/...965#post179665
https://www.homemadetools.net/forum/...518#post182060
https://www.homemadetools.net/forum/...076#post164837
https://www.homemadetools.net/forum/...986#post160486
https://www.homemadetools.net/forum/...908#post160181