Negative staining is a widely used technique for observing macromolecules and their assemblies by transmission electron microscopy. It is commonly employed to optimize specimens for cryo-EM. The stain, typically a uranyl salt, surrounds the structure, providing an outline view at about 20 Å resolution. Many macromolecules are relatively stable and rigid, and negative stain images provide a good representation of their structure. However, some are labile or flexible and their structure or assembly state is altered by binding to the carbon substrate on the grid before specimen staining. In these cases, the negatively stained appearance does not faithfully represent the structure in solution. This problem is reduced when samples are incubated on the carbon surface for short times (5 s) rather than typical times (30-60 s) before staining. To reduce disruption to a minimum, we have developed a rapid negative staining device (QuickStainer) using 3D-printed components, a stepper motor for precisely timed movements, and an Arduino-controlled interface to execute commands. QuickStainer produces consistent sample incubation times as low as 10 ms before staining. Tests show rapid adherence of molecules to the grid and greatly improved structural preservation of labile specimens compared with standard preparation protocols. The design of QuickStainer can accommodate inclusion of additional steps, such as timed incubation with enzyme substrate, before staining.