Personally, I'd re-paper with lining paper first to provide an evenly textured surface for painting.
On bare plaster you could use a thinned coat of paint to seal the surface before applying a top coat. Remember that where the exixting paint patches are much darker or lighter than the the new coat that they may show through more than one coat of new paint. Also, if the old paint patches are very smooth compared to the plaster, you'd probably still see where they are after applying the new paint. Re-papering first will overcome both of these problems..