Skimmg over a Painted wall.

I need to Skim a wall that has a Matt Paint finish over the original skim coat. It is in bits from years of hacking off wallpaper and badly in need of a fresh skimming.
Can anybody advise me on the correct preparation for the wall before Skimming. Do I scrape the hell out of it and also apply a strong coat of polybonding, ( any particular type of polybond?) and skim away or should the paint be completely removed somehow.
Im just concerned the skim wont take properly as the paint will act as a barrier between the new and old coats.
Any advice is welcome.
Thanks, Conor.

If the original paint is not flaking then sealing with Unibond may be the answer.

I've done a DIY job like that before. Got that skim over stuff from polycell (I think). Had that really glossy shiny paint on the walls and just skimmed over it. When I went to paint it, the paint pulled the skim coat off the wall. Needless to say I was not happy, I scraped the whole lot off, put on polybond and redid the whole job. It did work, but you have to be carful painting over it, if you paint too many layers at once without letting it dry, it might still come off.... so very tricky, but then I had this extra glossy hard paint on my walls, a normal emulsion I'd say should be fine - in some rooms I had stippled painted walls and I plastered over them no problem.