Dunno what all the fuss is about. I've done a helluva lot of DIY (still do!) and never had to wash down emulsion before putting more on, except where there's been a grease-mark on the wall. Dust-free surface, yes. With gloss - sugar soap, yes - it's important that the old surface is totally grease and dirt free, but thereafter, almost any modern gloss paint will take quite happily to the original gloss without the need for laborious sanding. Do the skirting first - you can wipe later emulsion drips off the new gloss paint, but you'll never be able to wipe later gloss paint drips off new emulsion!!