It's medium density polyethylene. It's only blue 'cos it's dyed. PE is colourless - your supermarket milk container is also PE. Blue for water, yellow for gas. We design landfill liners with high density PE - which has very nearly the same chemical resistance as MDPE to most things. You'll be OK to paint it with almost anything! PE is pretty unreactive - which is why most of the world uses it for chemical lagoons, landfills, contaminated land remediation, etc.