Methods and apparatus are disclosed for synthesizing a plurality of compounds such as biopolymers on the surface of supports. The synthesis comprises a plurality of steps in which reagents for conducting the synthesis are deposited on the surface of the support to form precursors of the chemical compounds and, ultimately, the chemical compounds themselves. An error in the deposition may occur in one or more of the plurality of steps. A reagent for forming the chemical compounds is deposited on the surface of the support. A determination is made as to whether an error occurred in the depositing of the reagent. If an error is detected, the support is treated to re-deposit at least some of those reagents that were not correctly deposited. In one approach the support is treated to stabilize precursors of the chemical compounds, the source of the error is corrected, and the reagent applied above is re-deposited on the surface.