This article is the first in a two-part series describing a process for conformal 3D printing on to inflatable substrates. Details for fabricating seamless, tubular elastomeric membranes by spray deposition on a double-curved air-permeable mandrel are presented, in Part I. The mandrels are created by casting gypsum into a desired form, and made permeable by applying pressurised air to the central core of the gypsum body during its crystallisation phase. The membranesin this case made from siliconeare created by spray deposition onto the mandrel using a constant surface angular velocity approach. These membranes are inflated so as to impart mechanical pre-strain in the rubber by stretching. The techniques described are particularly suited to fabrication of 3D printed Pneumatic Artificial Muscles (PAM) and Dielectric Elastomer Actuators (DEA). They can also be used to create removable substrates upon which a 3D print can be extruded, or alternatively integrated into a 4D print where varying levels of mechanical strain can be distributed through the various printed layers. Uses for the techniques described include soft robotics, stretchable electronics, biomechanical implants and custom bioreactors, particularly when combined with direct ink writing techniques.