Removing oxidation from my MGB bonnet?

I have a 1980 MGB Roadster. As I don't have a garage I have use a car cover. All winter it was covered with no problems then a few weeks ago to out for a run to charge the battery still no problems. Put the cover back, which is fairly heavy duty, took it of today and the centre of the bonnet has completely discoloured. The bonnet is made from aluminium. Anyway I have tried T-Cut and it will not remove it. Are there any experts that can help please.