Yes, under most circumstances, you can thin or reduce DiamondFinish Clear with KBS #1 Thinner or Xylene to thin DiamondFinish Clear back down to a standard viscosity. Doing this replaces the solvents that have evaporated off. Adding thinner or reducer can typically be done a few times to DiamondFinish Clear if needed.
Please check out the complete DiamondFinish Clear Application Directions before you begin your project.