" fiddlers would allow a dusting of rosin to accumulate so that, periodically, they could re-use that"

That doesn't work - the rosin welds itself to the varnish, becoming very difficult to remove, and possibly negatively affecting the tone.

It's really called 'laziness'...


With regard to oil for the body of the instrument - a good luthier told me - buy fresh walnuts form the store (they are several months old, having been dried anyway), good fresh ones you would want to eat yourself, take a few pieces of the meat, crush it, and use that oil to gently polish with a soft cloth.