I don't know how many bases and diorama frames I have screwed-up while building on them. Oh sure, I mask them and protect them as best I can, but somehow I always end up doing something to mar them. Now, I don't let my base get anywhere near the work in progress. Instead, I carefully measure and dry-fit my ground work substratum, usually styrofoam, or a piece of glass or plastic sheet, to make sure it fits exactly to the base, and then remove the base entirely until I'm finished, then mount the model to the base as the very last step.