I have tiled walls and brick at my washes. When I first bought them I spent a weekend deep cleaning them and since then every 2 to 3 month depending on the months of year we use a paint roller and roll acid on them and power wash them.
The first time is always a b*&tch because its probably been months if not years since the walls have been cleaned by the prior owner.
@Joswhaha what type of power washer are you using? I have 5000 psi unit with heat the does the job. We roll straight acid on the walls let it sit a few minutes then hit it with the washer. These walls had at least 20 years of build up and I know because we put up a warehouse 5 mins away and I would check in so often with the prior owner about selling and I know in those 20 years he never cleaned them.
Whatever you do I would not paint or epoxy them. I know owners/friends of mine that have and that is their biggest regret.