You can buy some timber slats to literally encase the four sides of the spindles to make them square. The timber can be glued and pinned to the existing spinles. For the correct thickness of timber you may have to get your local joiners shop to make it up for you and there may be some alteration required to the bottom string to accomodate any overhanhg from the new spindle covering. This should be a case of simply planting a new face on the existing face of the string. See our project on stairbuilding for help with the terminology.