Linseed oil is a traditional vegetable-based additive which is used as a binder and weatherproofer for LIMEWASH.  Its water shedding properties are considerable, generally a one per cent solution is all that is required in the final coat i.e. 50ml to 5 litres of limewash.  It is worth remembering that although a greater quantity of additive will increase the protection against the weather, it will also inhibit moisture movement within the wall.