An indirect answer to your question.
We have done 0.5mm pitch LQFP DIY boards.
We used 'proper' laser printable PCB Artwork Drafting film
We spent quite a lot of time calibrating the exposure time of the PCB in the UV box, and IIRC it was sensitive to a +/- 5 second variation. Too short or too long produced poor results. IIRC, we made a 'PCB' with some test patterns for different track/space distances to help us calibrate things.
We still had quite a lot of trouble getting good results. Then we discovered that the laser printer was stepping in, and trying to produce a 'grayscale' when the PCB artwork 'pixel' boundaries didn't match its own idea of pixel boundaries. When we examined the artwork under a microscope, we could see that edges were defined by a fuzzy (dithered) half-tone pattern, rather than a much denser, more uniform edge.
We improved the results by 'fiddling around' with printer settings.
Then I redid the footprint of the 0.5mm pitch LQFP part so that the gap between pads was slightly bigger. That gave better results.
Edit:
I know folks who have tried a 1,700GBP 'ebay' PCB mill. AFAIK they gave up due to difficulties getting consistent results. They have now spent a lot more to get a proper LPKF milling machine.
Edit2:
Is the entire board 'packed', with a need for 0.25mm track/space everywhere or is it mainly around the 80pin part?
Depending on where you are in your development process, and the sort of issues you are needing to fix, a way to reduce the pain might be to make a 'breakout' board for the LQFP part with your high-quality manufacturer. That would have lead-time, but once you have some, you might be able to turn-round the rest of the PCB using DIY.
It may also be the breakout PCB can solve some of your layout issues. If you put it's decoupling capacitors etc on its breakout PCB, its behaviour might be okay. My experience is manufactured vias are much smaller than DIY vias freeing up board area. Further, putting vias under the chip are awkward to do on a DIY PCB. So you might get a lot of benefit from the manufactured breakout, and hence make the remaining DIY PCB easier to route.
A traditional breakout usually has pins on 0.1" centres, in a square around the chip. You don't need to do that. You could use finer pitch connections and with pins in a non-rectangular, convenient, shape for your problem.
Maybe even consider doing a 4-layer breakout PCB, to make the rest of the board as simple to layout and make as practical.
The two pads that were highlighted are probably just the first ones that showed up in their rule-checker. If you changed just those pads and sent it back to them, they'd probably come back asking about the next two pads, etc. How wide are the current pads, and how wide are the component pins? It looks like a QFP package, so I wouldn't recommend removing the soldermask. That greatly increases the chances of solder bridges between the pins, especially when hand-soldering. I would say your best bet would be to decrease the width of the pads slightly (~.02mm total, or .01mm on each side) so that they can bridge the gap with soldermask. However, if the pins of your part are too wide to fit on narrower pads, you may not have much of a choice. Either eliminate the soldermask webs between pins and just be VERY careful about solder bridges, or go to a different manufacturer who can handle soldermask webs between pads that are 0.23mm apart or less.
Best Answer
We had a detailed discussion with the PCB manufacturer. Suffice to say, that they guarantee (according to their "Process and Capability Manual") a solder mask of at least 5um.
According to them, everything nearing the 5um thickness would already result in the "copperish" color shining through the solder resist.
The solder resists viscosity may vary slightly and cause differing results, especially at the edges, where some resist may flow off the copper.
Nevertheless we get better and more consistent results from other manufacturers, so it still stands that their processing quality may not be optimal.