Feasibility of coating plastic planters?

Hi! I have 3 giant plastic planters (maybe 25 gal) on my patio, which have grapevines happily growing in them. The plastic is getting somewhat sun- and heat-damaged. I was wondering if you experts think it would work to coat them with multiple coats of concrete, to a total thickness of maybe 2-3 inches?

If yes, I would really appreciate suggestions on how I might go about this, in terms of reinforcement, thickness of coats, etc.

Thanks!