The quest to develop materials that enable the manufacture of dimensionally ultra-stable structures for critical-dimension components in spacecraft has led to much research over many decades and the evolution of carbon fibre reinforced polymer materials. This has resulted in structural designs that feature a near-zero coefficient of thermal expansion. However, the dimensional instabilities that result from moisture ingression and release remain the fundamental vulnerability of the matrix, which restricts many applications. Here, we address this challenge by developing a space-qualifiable physical surface barrier that blends within the mechanical properties of the composite, thus becoming part of the composite itself. The resulting enhanced composite features mechanical integrity and a strength that is superior to the underlying composite, while remaining impervious to moisture and outgassing. We demonstrate production capability for a model-sized component for the Sentinel-5 mission and demonstrate such capability for future European Space Agency (ESA) and National Aeronautics and Space Administration (NASA) programmes such as Copernicus Extension, Earth Explorer and Science Cosmic Visions.