Discovery 2 chassis

Morning everyone My chassis is very rusty I have welded up the rear by the tank now most of the time chassis normally rot from inside out but mine has paint inside but the outside is very pitted has anyone experienced this I have been round it with a big hammer and plenty of oil it doesn’t really do much off-roading or wading I can’t understand why my chassis is very pitted on the outside but really good condition inside does anyone have any suggestions how to solve this thanks

Your chassis may have been protected well by the first owner having one of the various chassis treatments applied inside & outside very early in its life.
The external surfaces are not protected from the weather etc, so unless cleaned-up & re-protected regularly, will deteriorate possibly faster than the inside - although the fact that you'hve had to weld up the rear suggests the insides are rotting too.
If the car has been used extensively in areas close to the coast - ie salty air - the deterioration accelerates.