Actually, if you reread Marto's post you will see that he answered your last question:
"The grainy or fibrous nature of some types of old iron make a very rough surface especially if the blade been cleaned many times. Iron like this has open grain structure so it easy for substance applied to blade to penetrate below outer surface of the iron. Warangan will penetrate a little and give deeper etch, oil will also penetrate and sometime after old blade cleaned you can still smell the oil in the blade.
Actually iron like this not good iron because it shows that the maker of the blade did not wash the iron of impurity before using it to make the keris. "
Also Marto wrote:
"Also is kelengan iron . This iron very black and have a very rough open grain .
It is not good iron. "
Your logic on rusting perhaps makes some sense, though i'm not sure it would be correct to state that steel rusts "much" faster than iron. And i always thought sword makers moved into using steel not just because of increased strength, but also it's ability to hold a better edge than iron. But i could be wrong on that. Anyway, for many of us the regular oiling of our keris is not a thing of the past. I must say that i have been fortunate not to have seen any rust accumulate on any of my blades, new or old. Though it's a bit more work as the collection grows you just have to keep at it.
