Exterior painting can at the maximum be just a temporary solution of mild seepage in rainy season. It will not solve the real problem of leaking .
Leaking occurs in old buildings.That can be expected and then extensive re plastering is one solution.
In new buildings leaking occurs due to in sufficient curing, using low grade materials,and using wrong proportion of mixing plastering and concreting materials. Here the solution for leakage on side walls i can be installing canopies,overhead covering sheets,covering he exposed walls by water proof sheets.
If affordable, tiling the exterior walls can be done. That will not only solve the leaking but will also save recurring cost of painting and give a neat and beautiful look also.
But for very minor seepage or wetness without leaking,and if the wall area is less, then we can use waterproof emulsions specially made for exterior walls. But proper put work also to be don in the suspected areas before painting.