I remembered that when I had this problem I read about some thrips living their larvae stage on the leaves. Whilst sand could help resolve/solve this issue it might not cure the problem on it's own depending on what type of thrips you have.

"There are different thrips found on different plants - rose thrips and gladioli thrips have been with us for a long time, but the major threat to commercial crops at the moment is the more recent "Western Flower Thrips". The adults lay their eggs in a pocket in the leaf, and the larvae emerge to feed on the plant - growing buds and flower buds are particular targets. Young thrips (larvae) are similar to the adult, but without wings. The very young ones are white, and the older ones yellow-brown. The larvae live on the plant, but when they pupate the pupa often drops to the soil. The pupae may hide on the soil compost for several days or several months before the adults emerge, so remember that soil can be a source of infection."