most of the time yes you can just skim over it but it depends on how heavy the artex is. also if it is heavy on the walls then you will need a greasing of bonding before you put the finish coat on.now if it is heavy on the ceiling and the artex has been there along time such as the 70s, i would rather overboard it then skim.because in them days they would use white wash, and the plaster can react to this and drop off normally the last trowel, it costs more to overboard but at least your getting it done 100% right. beware also that some artex used years ago may contain asbestos so if you are thinking of scraping it back it may be worth while getting it checked out.It can take up to 3 to 4 days to dry depends how thick it is.hope this helps thanks tom.