Abstract: Improved reinforced plastic tanks and novel mandrel processes for producing them are disclosed. One aspect of the invention is concerned with an external support rib and a method for producing it. According to the invention, the rib is fabricated on a hollow rib form secured to the exterior of a tank with resilient netting and thread. The structure comprises chopped fibers and a resin which does not require a source of external heat to cure. The tank with the rib form secured to it is rotated as the lay-up is applied. Netting and threads are supplied to the rib form and they wrap around the rib lay-up and hold it in place until it cures.Also disclosed is a method for producing a double wall tank incorporating the novel rib. According to this aspect of the invention, the ribs are cured until they are at least tacky. Then, thin flexible panels are wrapped around the outside of the ribs and held in place by a wrap.