The paper is concerned with an unbalanced cylindrical rotor containing a small amount of fluid, spun out to form a thin fluid layer on the inner surface of the cylinder. The main interest is in the possibility of this fluid layer to counterbalance an unbalanced point mass, as well as in the derivation of approximate analytical solutions to the non-linear, forced equation that governs the fluid layer thickness perturbation. This equation is of the forced Korteveg-de Vries-Burgers type, and the analysis is based on the method of multiple scales. The leading-order solutions are capable of giving a qualitative explanation of the balancing effect of the fluid, in other words, to explain the mechanics of the so-called `fluid balancer'. Additionally, good agreement between theoretical and experimental results is found.