Taking part in the growing concern for repair and maintenance in STS, this article investigates epistemic dimensions of maintenance. Drawing on an ethnographic study of graffiti removal in Paris, it highlights the different objects of knowledge involved in this specific setting of urban maintenance and documents their relationships. It shows that, inspired by the 'broken windows' thesis, the anti-graffiti program that emerged in Paris at the turn of 2000 articulates three objects of knowledge-public order, graffiti and the citywhose intertwined definitions root a restorative maintenance epistemology. Such epistemology unfolds in an assemblage of policy documents, regulatory texts, contracts, technical specifications and procedures, information infrastructures and categories, removal techniques, tools and situated gestures, which take place in municipality's offices, contractors' workshops and during each intervention in the streets. The Paris graffiti removal program instantiates a preservationist approach which focuses on recurrent visual signs of disruption occurring on the façades and rests on both a distributed attention and a particular pace for interventions. It involves three main operations: measuring surfaces, identifying public expressions and composing with materials. None of these operations are neutral. Aimed at preserving a specific order, they also participate in the daily transformation of urban reality. The heterogeneous knowledge at play in maintenance practices intricately takes part in the becoming of the things whose stability it strives to ensure.