No, you're not missing anything Cassa. It's rather like your car. It's easy enough to paint it yourself, with a brush, but we take them to spray shops to get a perfect factory finish...........
If your guy had any pride in his work, he would have been the one to notice the sub-standard finish, and dealt with the sprayers. If I had been in his place, you wouldn't have seen them until they were right. You would never have known anything about it 'cos it would have made me look a prat.
I'd never let a job go sour like this. Very difficult to re-establish confidence now.
If you're up to it, do a deal to get rid of him. He's admitted fault by offering a reduction despite what he says. Give him a final payment with a promise of no comeback. I bet he'll take it.
Perhaps take a door to a car-refinishing place........... see if they'll do it. A quick rub down and a couple of decent coats should do it.
Oh, I'd happily have a word, but I'll bet you're nowhere near Exeter!!