τον γάιδαρο, όσο και να τον στολίσεις, άλογο δεν γίνεται

Proverb
(literally: as much as you decorate the donkey, he will not become a horse)


 * 1) you can't make a silk purse of a sow's ear, you can't polish a turd it is not possible to produce something refined, admirable, or valuable from something which is unrefined, unpleasant, or of little or no value