This depends on the condition of the existing clear-coat layer. If it is in good condition, then it is possible to just apply another coat. However, if the existing clear coat is flaking and delaminating, it is best to remove all paint layers, including the base coat. In a two-stage paint process, you need to apply the clear coat before the base coat is completely dry. If more than 24 hours have passed since the base coat was applied, you may not get a very good result.