When dirt and moisture sit on top of you spar in the wing root, it will form corrosion. It can be of two types, a light pitting or blisters that stand out and are deeper than they are tall.
The general run for pitting is that if any one pit after cleaning out all the corrosion and getting down to new material is to reject the spar if the depth of the pit is more than 15% of the spar wall thickness.
On Cheetahs this is 0.280 inches and on Tigers it is 0.303 inches. So 15% would be 0.042 for an AA5A and 0.045 for a AA5B. Usually one blister will reject a spar and multiple blisters seal the deal.