The interior decorative painting at Mission San Jose de Tumacácori is a rare survival of late 18th century-early 19th century artistic traditions of northern Sonora and the Kino mission churches. Despite earlier attempts to stabilize these finishes, the original painted lime plaster has continued to detach from the adobe substrate. Previous techniques to stabilize the paintings began with research by J. Rutherford Gettens in 1949-1952 and subsequent attempts in 1984 to reattach detached plaster have proven ineffective. The current research evaluates soil-based injection grouting in order to adhere the loose plaster on the nave and sanctuary walls. Earthen grouts were tested over the more commonly used hydraulic lime grouts in order to consider a more compatible system with the original construction materials. A well-designed earthen grout must be fluid enough to insure full penetration, exhibit low shrinkage and strong bond strength equal to its own cohesive strength for successful repair. Samples of the original adobe, mortar, and plaster were analyzed and local soils were sampled and tested in order to design a grout displaying optimal properties. The test grout was subjected to several geo-technical tests including viscosity, density, shrinkage, and expansion/ bleeding; as well as its hardened properties such as splitting tensile strength, capillary water absorption, water retention and permeability. The selected grout’s performance was finally analyzed with a mock-up assembly composed of friable plaster facsimiles and adobe, simulating 1/2" and 1/4" gaps. Half of the plaster facsimiles were consolidated with nanolime due to their friable nature based on recent parallel research. The research expands current knowledge on the use of earthen grouts for reattachment of earthen and lime plasters on earthen substrates.