AA's aircraft do not have any lacquer over it's bare metal finish. The alclad sheet that the aircraft skin is constructed from is protected against corrosion by a very thin layer of pure aluminium. This is on both sides of the metal and is very effective in the prevention of corrosion. However........If the pure outer layer is scratched or damaged, then corrosion or stress cracks can appear over a fairly short period of time, so this must be treated immediately by burnishing (polishing out), followed by crack testing and then an application of alodine (which chemically restores the corrosion protection).
Having an unpainted aircraft has a few advantages over painted:
Obviously the aircraft will be lighter in weight,but probably not as much as people might think, because the main weight in the paint on application is in the solvent or chemicals which keep it wet for applying it, most of these actually evaporate when the paint dries, so around 50% of the "in the tin " weight is lost.
It is much easier to keep an eye on any looming corrosion build-up or metal surface damage as it is not hidden under the paint.
Obviously there are no recurrent re-paints every few years which saves a lot of money! Although polishing has to be undertaken to keep the aircraft looking presentable as it can "dull" after a while.
Paint is susceptable to damage from hydraulic and engine oils, so no-touching up is necessary