We derive a finite-size scaling representation for the partition function for an Onsager-Temperley string model with a wetting transition, and analyze the zeros of this partition function in the complex scaled coupling parameter of relevance. The system models the one-dimensional interface between two phases in a rectangular two-dimensional region (x, y) ∈ℝ2,−L ≤y⩽L,o≤x≤N. The two phases are at coexistence. The string or interface has a surface tension 2KkT per unit length and an extra Boltzmann weighta per unit length if it touches the surfaces aty=±L. There is a critical valueac=1/2K and fora>ac the string is confined to one of the surfaces, while fora ťac the string moves roughly in the rectangular region. The finite-size scaling parameters are α=ac2N/L2 and ζ=L(a−ac)/ac2. We find that for |ζ| large, the zeros of the scaled partition function lie close to the lines arg(ζ)=±π/4 with re(ζ)>0. We discuss the motion of all the zeros as α changes by both analytic and numerical arguments.