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Realistically, what downsides would there be if first-world governments began banning spray paint? It is almost always used for criminal purposes nowadays.

You'd significantly decrease graffiti and with it the vandalism it entails. You'd also remove a source of intoxication for drug addicts chasing a high. I legitimately cannot think of a reason anyone would need canned spray paint other than to cause themselves or others trouble. Those who customize cars don't use cans, but their own high-grade applicators. Workmen might be slightly deprived, but we have shit like VR or UV/road paint if they badly need to know where to drill.