Spinach seeds have the reputation of being slow, spotty germinators and difficult to start indoors, but here's a technique that gets them up and growing after only a few days in the Seed Starting System. It's called "priming".  A week before planting your spinach seeds in the GSS, soak them in room temperature water for 24 hours, and then let them dry in paper towels for a day or so.  Place them in an airtight container for not more than a week, and then put 3-4 seeds into each grow sponge. You should get at least one seedling from each sponge, but if you get more than one, snip and remove all but the strongest when it's a couple of inches tall.