We have developed a high-purity sol-gel coating for the interior surface of glass cells used for polarizing He-3 by spin-exchange optical pumping. The coating is designed to minimize spin relaxation due to wall collisions. A longitudinal spin-relaxation time T-1 in a sol-gel coated Pyrex cell of 344 +/- 8 h was achieved, the longest T-1 we have ever recorded for a gaseous sample. Repeated trials indicated that the coating was quite robust. Results using an uncoated Pyrex cell were also quite good, although inferior to the performance of the coated cell. (C) 2000 American Institute of Physics. [S0003-6951(00)02839-4].