A 3-D computational plasma interaction modeling system is being developed to predict the interaction of electric propulsion plumes with surfaces. The system, named COLISEUM, Is designed to be flexible, usable, and expandable, allowing users to define surfaces with their choice of off-the-shelf 3-D solid modeling packages. These surfaces are then loaded into COLISEUM, which calculates plasma expansion from electric thrusters using a variety of functional modules. Functional modules are interchangeable, and can range from simple (collisionless ray tracing) to complex (PIC-DSMC). Surface interaction parameters such as ion flux, ion energy, sputtering, and re-deposition are computed. Development to date has progressed to include the two simplest functional modules: PRESCRIBED_PLUME, which imports and superimposes a plume distribution, and RAY, which performs ray tracing of flux from point sources. This paper presents a new COLISEUM algorithm for calculating equilibrium re-sputtering and re-deposition of materials. This algorithm enables calculation of net deposition and sputtering of surfaces inside HET test facilities as well as in the space environment Two cases are presented - one for a laboratory experiment in which sputtering and redeposition were measured, and another in which sputtering and redeposition on a generalized geosynchronous spacecraft is predicted.