Historic preservation is not straight forward, but it isn't that difficult. Time consuming, sure. But as the saying goes, anything worthwhile takes time to build.
Take for example one sentence from the National Park Service Department of the Interior's Standards for the Treatment of Historic Properties: "[a historic building] should be cleaned using only the 'gentlest means possible' to best preserve it." Simple enough, right? Well, it depends.
The reason why historic preservation is not clear cut is because there are unknown factors that dictate the construction process. Unlike a new construction project, or even a renovation of an old building, the repercussions of certain construction activities have to be evaluated. And thus, more careful attention needs to be paid to the means and methods of construction than in those other cases.
The National Park Service keeps a series of Preservation Briefs that describe in extensive detail the methodology and practices that are important to keep in mind for historic preservation. These are meant to be helpful in foreshadowing the outcomes. One in particular, Preservation Brief #6: Dangers of Abrasive Cleaning to Historic Buildings, was significant in our work to remove paint, spray foam insulation residue, and plaster dust from the interior historic surfaces at the Conley-Maass-Downs building.
We sampled a bunch of different methods to determine the "gentlest means possible." In fact, this experimentation was instrumental in determining our desired cleaning method. This is because different materials, even different applications of the same material, reacted differently to being cleaned. All together, we rejected two methods that damaged historic materials, found methods to be ineffective on some materials but not for others, and we accepted six methods which effectively cleaned the historic materials without any damage.
The results are stunning. We were able to effectively clean all of the materials to provide a more attractive finish for the interior environment without damaging the historic materials. That is the essence of preservation. It took a little longer, but it will make a lasting impression.