In August, 1942, Supermarine found a synthetic paint, which could give the required smoothness, with a matt finish.
As well as the new paint, it was agreed that the entire front 25% of the wing would have its rivet "divots" filled, together with all of its panel lines, smoothed down, then primed and painted.
Around 1941, a new trade, the Aircraft Finisher, was introduced, whose duty was to look after the paint surface. One no-no was the use of wax polish (not that some pilots took any notice,) since it made any touching-up, or repainting, extremely difficult.
The standard treatment for the paint was a rub-down with wet sandpaper, then a wash-down with clean water; any "expert," who says that they were in a glossy finish, needs a quick course in history. Once the authorities had found that the paint worked, Hawker and Bristol soon followed suit.
Russia got brand-new IXs, which had become surplus to requirements, due to the ready availability of the VIII, XIV & XVI; the fact that they didn't look after them should come as no surprise, since, when Rolls-Royce went to check up on how their engines were behaving, they found that they'd been tipped out, into the mud, and the Russians were living in the wooden crates.
Edgar