Method for manufacturing a motor vehicle external moulding ring, such as in particular a window surround or body shell moulding, made of aluminium alloy, by forming and brightening including steps for conventional degreasing, electro-brightening in phospho-sulphuric acid medium, ambient surface cleaning, rinsing, continuous current anodisation in sulphuric acid medium, pore sealing in two steps, cold with nickel and in hot water, of a sheet or strip prepared according to the following successive steps: a) Vertical continuous casting of a high-purity AA5xxx series alloy ingot, i.e. having a composition such that (% by weight): Mg ≤ 1.1, Cu ≤ 0.10, other elements ≤ 0.30, remainder aluminium of the AA5657 type, i.e. (% by weight): Si: ≤ 0.08, Fe: ≤ 0.10, Cu: ≤ 0.10, Mn: ≤ 0.03, Mg: 0.6 - 1.0, Zn: ≤ 0.05, Ti: ≤ 0.020, other elements < 0.05 each, and < 0.15 in total, remainder aluminium or of the AA5205 type, i.e. (% by weight): Si: ≤ 0.15, Fe: ≤ 0.7, Cu: 0.03 - 0.10, Mn: ≤ 0.10, Mg: 0.6 - 1.0, Zn: ≤ 0.05, Ti: ≤ 0.05, other elements < 0.05 each, and < 0.15 in total, remainder aluminium or of the AA5505 type, i.e. (% by weight): Si: ≤ 0.06, Fe: ≤ 0.04, Cu: ≤ 0.01, Mn: ≤ 0.03, Mg: 0.8 - 1.1, Zn: ≤ 0.03, Ti: ≤ 0.010, other elements < 0.05 each, and < 0.10 in total, remainder aluminium or of the AA5210 type, i.e. (% by weight): Si: ≤ 0.06, Fe: ≤ 0.04, Cu: ≤ 0.01, Mn: ≤ 0.03, Mg: 0.35 - 0.60, Zn: ≤ 0.03, Ti: ≤ 0.020, other elements < 0.05 each, and < 0.10 in total, remainder aluminium, b) Reheating of the ingot at a temperature of 480 to 530°C for at least 1 hour, c) Hot rolling and cooling, d) Cold rolling including intermediate annealing in a continuous passage furnace, i.e. holding between the solvus temperature and the burning temperature of the alloy for typically 3 s to 5 min followed by air or water quenching. a) Vertical continuous casting of a high-purity AA5xxx series alloy ingot, i.e. having a composition such that (% by weight): Mg ≤ 1.1, Cu ≤ 0.10, other elements ≤ 0.30, remainder aluminium of the AA5657 type, i.e. (% by weight): Si: ≤ 0.08, Fe: ≤ 0.10, Cu: ≤ 0.10, Mn: ≤ 0.03, Mg: 0.6 - 1.0, Zn: ≤ 0.05, Ti: ≤ 0.020, other elements < 0.05 each, and < 0.15 in total, remainder aluminium or of the AA5205 type, i.e. (% by weight): Si: ≤ 0.15, Fe: ≤ 0.7, Cu: 0.03 - 0.10, Mn: ≤ 0.10, Mg: 0.6 - 1.0, Zn: ≤ 0.05, Ti: ≤ 0.05, other elements < 0.05 each, and < 0.15 in total, remainder aluminium or of the AA5505 type, i.e. (% by weight): Si: ≤ 0.06, Fe: ≤ 0.04, Cu: ≤ 0.01, Mn: ≤ 0.03, Mg: 0.8 - 1.1, Zn: ≤ 0.03, Ti: ≤ 0.010, other elements < 0.05 each, and < 0.10 in total, remainder aluminium or of the AA5210 type, i.e. (% by weight): Si: ≤ 0.06, Fe: ≤ 0.04, Cu: ≤ 0.01, Mn: ≤ 0.03, Mg: 0.35 - 0.60, Zn: ≤ 0.03, Ti: ≤ 0.020, other elements < 0.05 each, and < 0.10 in total, remainder aluminium, Mg ≤ 1.1, Cu ≤ 0.10, other elements ≤ 0.30, remainder aluminium Si: ≤ 0.08, Fe: ≤ 0.10, Cu: ≤ 0.10, Mn: ≤ 0.03, Mg: 0.6 - 1.0, Zn: ≤ 0.05, Ti: ≤ 0.020, other elements < 0.05 each, and < 0.15 in total, remainder aluminium or Si: ≤ 0.15, Fe: ≤ 0.7, Cu: 0.03 - 0.10, Mn: ≤ 0.10, Mg: 0.6 - 1.0, Zn: ≤ 0.05, Ti: ≤ 0.05, other elements < 0.05 each, and < 0.15 in total, remainder aluminium or Si: ≤ 0.06, Fe: ≤ 0.04, Cu: ≤ 0.01, Mn: ≤ 0.03, Mg: 0.8 - 1.1, Zn: ≤ 0.03, Ti: ≤ 0.010, other elements < 0.05 each, and < 0.10 in total, remainder aluminium or Si: ≤ 0.06, Fe: ≤ 0.04, Cu: ≤ 0.01, Mn: ≤ 0.03, Mg: 0.35 - 0.60, Zn: ≤ 0.03, Ti: ≤ 0.020, other elements < 0.05 each, and < 0.10 in total, remainder aluminium, b) Reheating of the ingot at a temperature of 480 to 530°C for at least 1 hour, c) Hot rolling and cooling, d) Cold rolling including intermediate annealing in a continuous passage furnace, i.e. holding between the solvus temperature and the burning temperature of the alloy for typically 3 s to 5 min followed by air or water quenching.