We are often asked "Do you need to putty between the boards? "

If the gaps are wide and you can drop a card or 5 cent coin through, then it is best to close this gap from underneath (if you can). Filling from the top only is usually not sufficient as once the boards move through use and temperature fluctuations any filling can come loose and fall through too. Where as if the gap is closed from underneath then any loosened putty will be able to be supported .
 If there are only thin gaps, we advocate that is best to leave them as they are. This allows the timber easy natural movement , however  this will then depend on the coating you have (or will use) too as many coatings that remain on top of the boards will restrict this movement also. As penetrating oils this is another bonus of the Livos oils in that they do not restrict natural movement  of the timber.
There are many forms of putty on the market, ranging from water based through to resins.  We use timber mate putty  as it has a porous texture so is able to absorb the oils in a similar way to the timber . It leaves a more consistent sheen level over the floor, also one can mix colours together if need be to get a perfect match or use a contrast color for impact.
We would always recommend you do a test sample as when the putty is dry, or coated, the color will be affected, often it will become darkened.