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Most brass doorknobs are coated in lacquer or a polymer.

It appears that COVID-19 lasts for about 4 hours on copper. So better than other surfaces, but it does still live for a while

source: https://www.bbc.com/future/article/20200317-covid-19-how-lon...

I wonder if you could just spray paint them with metal particles.

From what I've seen, yes that should work. It'll wear off much faster than either a solid metal piece or electroplating simply because it'll be a thinner coat but it would otherwise work.