The protection of silver nanoprisms against etching by the epitaxial deposition of a thin layer of gold in solution has been investigated. It has been found that at low Au/Ag ratios (similar to 0.08 to 0.17) a thin layer of gold is deposited on the edges of the nanoprisms as expected, but without the structural damage typically associated with galvanic replacement. Furthermore, this layer of gold provides robust protection against etching of the nanoprisms by chloride and is strong evidence that etching by chloride is face-selective and does not take place at the flat {111} faces of the nanoprisms. Additionally, the deposition of a protecting layer of gold results in only a small red shift in the position of the main plasmon resonance. We have investigated the sensitivity of the localized surface plasmon resonance (LSPR) to changes in the bulk refractive index of the solution and find that the gold-protected silver nanoprisms are promising candidates for the development of new refractive index-based biosensors.