Alan, lemon juice gives a brownish, yellow color and is indeed for staining not very suitable. Limes give a more clear juice that won't discolor as a way of speaking.
Strained and filtered juice in combination with a stiff brush is more convenient to use indeed. Using the raw fruit i learned from the fathers from my friends here in Holland. In my childhood i grew up in a neighbourhood with many indonesian people. I had a lot of friends among them. I suppose the fascination for keris and malay weapons was lighted there
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The reason for using raw fruit was that they could leave it on the blade on a spot with rust that wasn't removed during the rubbing. The fruit soaked the rust spot that came of more easier a while later. Could be a local habbit that was brought to Holland those days?