TotalBoat Thixo can be applied directly to a previous application of Thixo if it is still tacky. The only main consideration is when filling larger gaps, do not apply more than a maximum thickness of 1/2". If the Thixo has cured completely, it needs washed with a mild soap and warm water, then sanded to give a rough surface before reapplication.
This applies for any of the Thixo product line.