Sue I hope you'll try this out on your polypropylene trash bin first to make sure it's not a type of coating that will flake off. You've already done your time with flaky cladding treatments! Who knows what's in this stuff - could be 10 cents worth of boiled linseed oil, a flattening agent to soften the shine and a pinch of soot from the baghouse of the local coal-fired boiler to darken things up a bit...
As they say - test in an inconspicuous area!