The sanding and finishing of white pine floors is a fairly new innovation.What many people forget is,how many types and colours of pine there are, Oregon and pitch being two of the more common examples, and both beautiful to look at,if carefully sanded and finished professionaly or otherwise.The string idea is one I have used successfully,though I used it with PVA glue,before filling.I would not use any kind of mastic for floor gaps it can get dragged out and leave smears.If the floor has been in situ any decent length of time,it will have expanded / shrunk till it's limits (barring floods) so a hard plasticised filler should be O.K.