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I've got a press-fit frame with a bit of a ropey BB face, numerous small nicks in it. It takes a PF30, plastic sleeve affair.
Is facing good practice on a press fit like this, or is no one arsed? Given the plastic sleeve must be deformable to an extent, wondering if it would make any difference.
I wouldn't
If you're careful, a quick spin with a lapping wheel will smooth things out a bit.
I had to smooth the edge of a 'step' inside my BB shell as the Hope BB92 LHS cup I was trying to install is longer than the plastic Shimano/SRAM type.
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its f@*d mate. best just scrap the frame and replace with a threaded bb shell frame.
Press fit I can understand maybe a benefit. Threaded external BBs I've never seen the point of it. If the threads are true and aligned then external BBs are going to screw in aligned. Doesn't matter about a blob of paint (it will probably squish out anyway).
Dunno when some of the cheap shite out there can't even get the seat pin to a recognised or uniform diameter who's to say it's just paint and that the faces of the shell are parallel.
Tighten that up threads or not all you want but there will only be one loser in that situation and in will end in non parallel bearings and side loading.
Unfortunately the number of times I've had to cut anything off a bb shell to make em parallel is minimal and it wasnt always limited to the cheapest of stuff but always good practice to check because I had the tools at my disposal.
In the real world without the tools I'd only check it if I was having premature bearing failure if it wasn't obvious at build stage.