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i have white salt like stuff on my wall inside living room in the corner and just above the skirting board and about 1 foot to the left of the window.

Its not alot of white furry stuff only a little,but its not doing the paint any good !

Walls have been painted with watered down matt paint then acrylic eggshell with added pva.

Walls were skimmed with new plaster about april time.
House is empty and has no heating on since its been plastered/skimmed.

So since april no heating and now its been painted all the house is fine except the bit mentioned above which where the paint has blistered and peeled off.

Were talking of an area about no more than 2 ft square.

Any idea's what it is and what can be done?

I have no problem anywhere else in the house

its called efflorecense, salt deposits from newly plastered walls the salts will dissipate with time, brush off the salt and ventilate the room leave for a week and then repaint.

My painter has sanded the wall slightly,undercoated,left a heater on it for 2 hours and then re-painted.

If paint starts peeling again what can i do?
Plaster is surely dry now as ive had it done in april (5-6 months ago)

Or is it that not much ventilation has been in the house as its been empty for over a year with no heating on.

efflorescence is the white stuff you see on new external brickwork after it rains, it goes eventually.
The hairy inside stuff is salt crystals growing from ground salts which have come into the walls and plaster from outside via the ground moisture. Its a sign of damp penetration. Even if you seal off the damp ingress by having a chemical dpc inserted, the salts will still be there and as they are hygroscopic they will attract atmospheric moisture and continue to grow, even through new paintwork. Moist areas like loos and bathrooms are the worst. We had our WC completely redecorated after this same problem including partial replastering but it has still come back because the insurers wouldnt pay to have a dpc inserted or to re-plaster the whole area.
1- find moisture source and eradicate
2-hack off plaster to 18" height
3-replaster in sand and cement with a salt retarding additive -builders merchants have it