Plaster/sciming of the walls is now dry after letting it dry naturally,noticed a small wet patch (only slightly damp) on inside wall in the corner,its now got the salt appearing,im ready to paint, should i put damp seal on it first then under coat and paint or just use damp seal and paint over?
What you have is effloresence. It is caused by the salts coming out of the plaster as it dries. No you can't just paint over it. You have to eradicvate the source of damp first otherwise it will keep coming back then when it is dry brush it off first. I always prime an area that shows this effect with either Zinsser or a spirit based undercoat after the brushing off stage.
So to the source of damp:
When you say plaster and skim do you mean bonding coat followed by topcoat or are you just referring to a skim over plasterboard or old plaster? The former can take up to six weeks to dry out properly whereas the latter is a matter of 24 or so. Which is it? How thick was the plaster at the damp patch? How long has it had to dry?
Whilst a DPC failure is a possibility I think it would be wise to consider other possibilities first which may be more likely if we are certain the plaster is completely dried out. Are there any pipes in the wall at this place? How close is it to kitchen or bathroom?
The other thing is how high up the wall is the damp patch, If it's above about a metre it could be a an ingress of water from elsewhere. i.e. pointing or roof coping. But sometimes it can take up to 48 days for a plastered wall to dry out that's assuming it's been bonded and skimed.