Cleaning and stabilising

Start dry

Begin with a soft brush and a dry cloth and see how much of what you are looking at is simply dust and loose dirt. A surprising amount of what reads as a ruined finish is a layer sitting on top of an intact one. Photograph the piece before you start, because judgement drifts once you are half an hour into a surface.

Escalate slowly

If dry work is not enough, move to warm water and a mild detergent on a cloth rather than a flood, and dry immediately afterward. Only then consider a solvent, and only after testing it somewhere that does not show. Every step up in aggression should be a step you decided to take, not one you drifted into because the previous step was slow.

Leave stable corrosion alone

Not all corrosion is damage. A tight dark oxide on bronze or brass is protective and is part of the object's history. What matters is whether the corrosion is active: laminating rust that is jacking a joint apart, or a powdery bloom that returns after cleaning, is a problem to address. A stable dark surface that does not come off on a cloth is not.

Stabilise before you improve

Fix what is failing before you make anything look better. That means arresting active rust, consolidating a lifting finish, and supporting a cracked casting. Cosmetic work done over an unstable substrate has to be done twice, and the second time is always harder because the first attempt is now in the way.