The ISS has quite a large area exposed to space, and is subject to both the solar wind and micrometeoroids.
Protons and other nuclei in the solar wind can both sputter surfaces causing permanent mass loss, and implant, causing mass gain which may be either temporary or permanent depending on several details (species, energy, nature of the surface).
Various other particles impinge on surfaces as well.
Likewise micrometeoroid and meteoroid impacts will both add some mass and will disperse some as well (analogous to ejecta).
Are there any theoretical estimates of mass gain or loss of the ISS due to each of these processes? After two thirds of a billion seconds over several thousand square meters of surface area, could it be of the order of a gram? A kilogram?