If you don't want it satin or eggshell could you not ask him for a flat matte spray finish?

First of all your painter did not do what you wanted! You can do a flat finish if wanted by spray paint, or go with brushing the paint on. Your railing is short so you should do it yourself. It will cost you more if bring in a new painter. I come from a family of painters.

Ken

"Right . Most of the time people do a powder coat. But I assume that's impossible for us?"

Do you understand what powder coat is?

Item needs to be bare metal. It's sprayed with a powder, then baked in a large oven.

You could uninstall, remove paint -- sandblasting works well, then powder coat.

Yes I know what powder coating is and it would be the BEST. I do soda blasting and it takes awhile to take off. Plus you will look at a large bill to do it at your home. Depends on your wants and needs for the look!!

Ken

No need to repaint. You just need a clear coat in flat. Less mess

It's a gorgeous railing. I would do some research in order to get the look you desire. I recommend calling a local powder coater, even if it is already painted. Just make them aware of the situation. If nothing else they might have some great solutions for you. We recently had a large display done and I was surprised at how affordable it was.