I painted mine 3 seasons ago and not one flaking problem.
Here's what I did.
I got a red scotch brite pad and scuffed the whole post.
I then applied some clear followed by some black.
The black paint can had some plastic patio furniture on the label. I'm not sure if they still make that specific paint. I bought it all at Home Depot.
Scuffing takes the gloss off and the clear paint adheres to the post like a primer.
Back in the day I used to race remote control cars and we always painted the underside of the clear plastic bodies with clear 1st, then our choice of color.
So I just brought that idea with me. Not sure if it's necessary but it works great no the less.