Useful advice, delivered too late

Jonathan Swift

You can't make a silk purse out of a sow's ear.

Alexander Barclay 'Certayne Eglogues, 16th century'

None can make goodly silke of a gotes fleece.'

Stephen Gosson 'Ephemerides, In 1579'

seekinge too make a silke purse of a Sowes eare.

[From "The Dictionary of Cliches" by James Rogers (Wings Books, Originally New York: Facts on File Publications, 1985)]