The hobby machinist community has been buying cheap Chinese milling machines and lathes for decades and disassembling them to fix poor finish and assembly (replacing bearings, scraping or polishing bearing surfaces, better fasteners, etc.) and then reassembling to result in a much higher quality machine at a reasonable price. Could something similar happen with watch movements?

It not only can be done, it routinely is.

Especially with the Chinese watches, they often come from the factory with less-than-perfect attunement. Watch repairmen will take a watch like this, clean everything, maybe replace a bearing or two, use high quality lubricant, possibly even swap a gear or so if it needs it.

They can also take an older mechanical in pretty bad shape and restore it to health. There's a point of no return for any mechanical device, of course.

My Omega Seamaster gains ~2-3 minutes a month (so roughly 4-6 seconds a day.), it's rather annoying but I understand that it's pretty common for an automatic mechanical watch.

> My Omega Seamaster gains ~2-3 minutes a month (so roughly 4-6 seconds a day.)

That falls within the bounds of accuracy required of a Swiss certified mechanical chronometer (that is to say, a clock accurate enough for marine celestial navigation) and is pretty good.

As a marine engineer I have a genuine and real appreciation of mechanical chronometers, though any working watch/clock I use will always be quartz as for accuracy and price, nothing else comes close.

https://en.wikipedia.org/wiki/Marine_chronometer

For something mechanical operating under random g forces and over a wide temperature range, that accuracy is truly astonishing.

Fully agree, automatic mechanical watch movements are truly a marvel of engineering. The fact that the movement is powered purely by the kinetic forces naturally applied from wearing it, and can reasonably accurately keep time over years, under a wide range of conditions… is truly impressive.

Did you have it regulated? Asking because my Tudor Heritage Black Bay 79220R uses the same movement, but will lose around 30 seconds in a week... I mostly wear it when I'm not all that active (e.g. in the office, out about in the city, etc.), but it still seems too much to me.

Loosing 4 seconds per day (28 per week) falls within the bounds of accuracy required of a Swiss certified mechanical chronometer and is pretty good.

https://en.wikipedia.org/wiki/COSC

[I made a similar comment further down.]

Nope... came from Germany that way.

As other have notes though losing ~4 seconds a day is pretty accurate all things considered. Seiko's 3Hz movements can gain or lose _half a minute_ a day and still be considered within spec.

That seems surprising bad. Maybe not strictly out of tolerance, but definitely something that could be considerably improved by having the movement regulated (or perhaps serviced, if required). Is it still under warranty?

Not from the manufacturer, no. I bought it second-hand and the seller does offer a 36-month warranty for any mechanical dysfunctions, though I'm not sure whether something like this would qualify. Even if it did, I'm not all that enthused about bearing the costs of sending the watch to way across Europe if it could just be regulated by my local watch shop instead. Am I understanding correctly that if I had it regulated, then it the deviation would be minimal for years to come or how exactly would that pan out accuracy-wise?

I have swiss (ETA based), Japanese (Seiko) and Chinese watches. I think one of the Chinese watches I own is the most accurate and reliable of all of them.

which model chinese watch?

It's a sea-gull aqua Terra homage. With a sea-gull movement.