Best filler for a hair line crack?

That's what I normally use, and I believe one can squeeze the filler quite a way into the crack. But with something like a cake pump a much deeper fill might be achieved, somewhat akin to pumping the stuff in from a hypodermic syringe. (Perhaps someone knows whether the latter is available from vets, say for injecting horses, as the needles my practice nurse uses for taking a blood sample is far too fine for Polyfilla.)

As the crack no doubt extends right through the plaster to the brickwork behind, if only one could find a way of pushing the filler right back to the brickwork, that would in my view be a very successful result. This is not possible with a filling knife. With the knife I would expect the filler to reach a depth into the crack of no more than two or three millimetres at best, instead of the full depth of the plaster.

MM

I've never had filler come loose. With a real hairline crack you have to rake it out to get any appreciable (aka useful) amount of filler into it. A crack 1mm wide is not "hairline" it's a real crack. I might just run the tip of a trowel along it to make sure there are no loose edges but I wouldn't open it up any more.

I don't think it's worth filling them - if they're moving, it's a pointless exercise, anyway. I have, with ceilings, injected dilute PVA into hairline cracks, as mentioned before. It might be a good technique for walls, too.