Paint flaking on garden planters help with primer

Hi there, a while back we bought 2 square planters which are lightweight moulded pots made of something similar to glass fibre or GRP. So the paint started flaking off and I have tried to wire brush, rub down and re-paintbwith some Matt black exterior universal paint. That flakes off in no time as well. I'm thinking maybe I need a high performance primer. Any suggestions pls?