I am going to replace my square tapered spindle type bottom bracket with a Hollowtech 2 external bearing one.

In its simplest form, I guess this will involve removing the old one and just thread in the new one - but I've seen some articles that suggests that the bottom bracket shell faces must be machined.

Is this "in rare cases" or should the shell faces always be machined before fitting an external bearing one? Even if the frame is not new?

You haven't said anything about changing crank, so you realize that the type of BB you have is determined by the crank, not by what you like?

For machining, for this process, it's called facing, and it ensures that the BB ends, are positioned parallel with each other, is it worth doing? probably, but the chances are you will wear out an external BB well before you get any issues from not having the BB shell faced, however would always get this done if using a high end BB, would expect high end frames to come pre faced from the factory.

Yes, I have a external BB crank to go with it as well :-) Thx for your reply.

You can check for the need to face a BB shell. First remove all paint from the faces. Screw the cups in until they just contact a .010 inch feeler gage, then use .008 and .012 feelers to look for high or low spots around the face. If those limits are exceeded, you need some facing done. The idea is to make the faces square with the threads, in addition to being parallel to one another.

When I switched several bikes over to Campy ultra-torque cranks, I found none in need of facing and all of the BB shell widths were very close to the nominal 68mm. With the ultra-torque system, the BB shell width must not vary more than plus or minus .8mm.

What I think I will do it to let my LBS remove the old BB and tell them to "make it ready" for fitting a new one. Then they can see if it needs to be done or not.

Is it required to remove the painting on the shell face before fitting the BB?

A quick check for alignment is to thread the cups in hand tight and then try to slide the crank spindle through them. If the spindle slides through both with no apparent binding your bb shell is good. If it has to be forced in, the shell should be faced. And, yes, the paint should be removed.

Hand tightening the cups won't tell you a thing. Some cups have thread locking compound applied to the threads at the factory, so hand tightening is no even possible. It takes some torque to force the cup against the face. If the threads are not aligned on both sides, that can't be fixed. Facing just makes the faces square to the threads, reducing the chance that a cup might come loose while in use.

There should never be any paint left on a face when a cup has to be tightend against it.