Positronium (Ps) formation by weakly localized long-lived electrons and positrons at low temperatures successfully explained the Ps formation enhancement at low temperatures observed in polymers and molecular solids. The phenomena expected from this Ps formation mechanism, such as an effect due to visible light exposure, the linear relation between the density of weakly localized electrons and Ps formation and delayed Ps formation were observed. Some possibilities for application of this new Ps formation, such as a new idea for observing spin polarized positrons, will be given. Weakly localized long-lived electrons can be polarized when they are placed in magnetic field at very low temperatures. It is probably possible to see the polarity of positrons by Ps formation with these polarized electrons. The detail of the idea will be developed.