Exposed plaster treatment?

Chip off the plaster and treat the wall

There are some sealant witch you can use, but I highly suggest too chip off the plaster and render the wall using K-rendering materials or for a cheap version use a cement based rendering.
Plaster is well known as a water sucker... I really suggest to get it removed from the outside wall otherwise the water will penetrate inside your property and cause serious damages to the property and also to your health developing mould on the damp surfaces.

Plaster is only to be used for interior… if exposed to elements of rain etc you will get damp problems… not worth the chance of using paints etc… do a proper job and remove completely back to brick and re-render… hope this helps

If the plaster is not coming off, then you can first pent whith PVA, and then treat the wall

of course there are suitable paints that will protect the plaster, in each warehouse they will advise which paint to use best

Best to get rip of plaster and render the area of old work