rust damage in 212 block

So,i have a good block from a 37T,only bored out 0.05,with about 0.06 wear,so an 0.020 rebore should be fine,however there is a fair amount of corrosion in the water chambers,with some deep pitting on the external cylinder walls.Does anyone have knowledge/experience of this problem,i.e,are pinholes into the cylinder a common occurence? i dont want to spend a lot of money on an engine that leaks into the cylinders,how often does this happen and how can one check the wall thickness?