The Handyman's Pocket Book: Creosote.

Creosote which has become thick through standing should be thinned with paraffin oil.  To clean hands use paraffin oil.

It is risky to paint over creosote as the latter is liable to bleed through and cause discolouration.  If the creosote has been on for several years it may have become impoverished, and it might be possible to use paint after two coats of knotting.  Another alternative is to use aluminium paint priming.  There is always an element of risk, however.