You can buy the ceramic coal paint spray from places like B & Q to give you're coals a new lease of life.
You're question should be why is my fire sooting up ! This is a sign of incomplete combustion and oxegen starvation and suggests the fire is in distress and possibly producing the leathal fumes. Check and make sure any air bricks and porpose provided ventilation has not been covered up or is blocked and also get it serviced and checked for safety ASAP.
That is a shame as my local B&Q sell it but it is one of the large mega-warehouse stores so maybe the smaller ones do not do it.. Glad to here it is not sooting up.