Protecting brick surface from water and frost damage

I live in Norfolk and have an old barn near the coast (about 500 meters: I only mention this as salt mist may be relevant to the discussion). The barn is built with quite soft Norfolk brick and lime mortar. some are quite crumbly. I will be repairing the building over the next 3 years, replacing some brick, where they have determinate do away and reprinting with hydraulic lime mortar.
My question is that much of the building will still want to absorbe a lot of water as the old bricks are so soft, so is there a solution I can use that’s ‘clear’ so as to create a bonded layer over the brick surface (and mortar) to protect them from further water and frost damage yet maintaining it to pass the maximum amount of water vapour. I do not want to trap moister in the building fabric.
Your advice will be most welcome.