In a previous paper we developed a generic modelling framework, for the diffusive mass transport through the turbulent boundary-layer of multi-component, reactive flows where one or more species may deposit on the wall. In the current paper, we describe the implementation of the modelling framework as a mass transfer wall function for coarse grid CFD modelling of direct precipitation fouling. The wall-function is formulated as a sub-grid model, where the simplified 1-dimensional governing equations are solved perpendicularly to the wall. This enables us, in principle, to model complex heat-exchanger geometries in two or three dimensions, since we reduce the computational cost of resolving the fine length-scales at which the boundary layer mass-transfer is determined. The mass-transfer wall function is implemented as a user-defined function (UDF) in the commercial CFD software ANSYS Fluent. To demonstrate the capabilities of the wall-function, it is employed on an axisymmetrical reactor geometry wherein a semi-artificial, 4-component, ideal mixture of perfect gasses flows. One of the components, namely the reaction product of two of the other components, can sublimate and deposit on the walls of the reactor.