Limewash is less tolerant of a poor substrate than paint, because it relies on the brick drinking it in. The wall needs to be clean, sound and absorbent: dirt, moss and loose material are washed away, efflorescence (white salt deposits that point to moving moisture) is noted, and a splash of water shows whether an old repellent sealer will block absorption.
Underlying problems come first, whatever the finish. Open joints, split sealant, overflowing downspouts and earth piled high against the base get repaired or flagged first, since breathability helps a wet wall dry but cannot stop new water arriving. For pre-1978 homes, removing old paint by scraping or stripping is regulated work: Washington’s renovation rules require lead-safe practices from a certified firm.