WHITE FLOURY PATCHES ON GARAGE FLOOR EXTREME DUST WOT IS IT?

I want to give my garage a clear out and a new lease of life, the floor has been painted by previous owner and obviously quite some time ago, in garage floor paint. It is a concrete floor. I thought I would first loosen any of the flaking paint and there are parts where this paint has come off. On brushing I notice the white greyish patches where the paint has come off are just causing continuous dust. I mean I could brush forever and still remove dust, its not a dirty dust but seems almost white and floury. Will this just need PVAing before I repaint in order to seal, or is this something more sinister. Once again your help on this site would be much appreciated. thanks.

if it is fluffy white powder it is probably efforescence.

If it is at all gritty it will be the concrete surface wearing and making dust. You can brush a concrete floor forever and it will never stop making dust (until it is worn away to nothing).

It seems to be a bit fluffy white so probably efforesence, but also seems as though the concrete floor as you say could be brushed forever.
What is the best thing to do to treat this before I paint with floor paint. should I just brush on some diluted PVA on all the bare patches?

If concrete is "over wet" when it is poured, and the excess water forms puddles on the surface, it makes that surface weaker because the cement in the concrete in those areas would have become diluted/watery, leaving you with a powdery/weaker surface. Most concrete on major contracts/jobs, would be slump tested to ensure the correct amount of water had been added. With the PSA, on Government Contracts etc, if a load/s of concrete were "over wet" and failed the slump test, the load/s were condemned because the future strength of the concrete itself had been compromised. Even a semi dry sand and cement floor screed with the odd "over wet" batch here and there, would also give a powdery, weak surface in those areas where the excess water had come to the top. Too much water in or on, concrete, cement screeds, mortars, plasters etc, is not good. It might be easier to work with, but gives a far weaker end result, and all the problems that go with it.

Roughcaster.