We have a (formerly) white plastic strainer in the kitchen sink that has acquired a brown biofilm layer. Bleach is moderately effective, but the surface is just ooky.
Green tea is suspected, but the evidence is not, at least according to me, conclusive. More research is in order.
Took the evidence to the Basement Laboratory’s Machine Shop WIng and skim-cut both faces, cleaned up the rim, drilled out the holes, countersunk the holes to get rid of the chaff, and it’s all good. The surface is probably too rough, but we’ll see what happens.
I figure I can do that maybe twice more before I must make a new one; looks like a perfect match for CNC, doesn’t it?