What do you use to coat a tar and asphalt flat roof?

Depends on the value of the building and your reasons and intentions. If it's a high value building, say a commercial or residential structure, with a long life span wanted, I'd use a SBS modified bituminous roofing membrane. It will seal the rood for 20+ years if applied correctly. For a lower value roof, say over a garage or storage shed then there are several hot applied roofing coatings available. Any competent roofing company can advise you. The exact one will depend on the original roofing material (a cutback or an emulsion). If the roof has had leaks in the past and the insulation is wet you have to be careful not to trap moisture inside the roof or you will have some long term problems. In any case this is not do it yourself job. There are some special equipment and techniques required because working at heights and with extremely hot flammable and sticky liquids requires some training and safety techniques to be done safely.