You need to source the cause of the bald spots first. Pet urine? Grubs? Moths (hope you do not water late in the day?)? Disease? Just gets too hot and the roots are too shallow to survive the heat? Overwatered? Mowing too short overall? Soil is out of balance? Fertilizer burn?
You might want to go for "Carpet Grass Seed." In PA i'm guessing you want something that can sustain a good bit of moisture, and withstand the winder cold weather, correct?
Carper grass seed comes in tons of different names, but it should all do the same thing, if you spread this seed around pretty densely, and water it once or twice, it'll take and grow like mad. It isn't the most attractive looking grass, and has a similar look to crab grass, but it'll grow anywhere once you get it started, and it'll move. Fast.
We used it for our sheep We have three pastures for them, and while we have them in one for about two weeks, by the time those two weeks are up, the carper grass has gotten hold in the first location, and when we go from the third back to the first its well "over-taken" the previous grazing area.
Give it a whirl :P If you really dislike it, it has shallow roots and you can mow the grass very low and just poison it.
It's a quick-fix solution to grass, but doesn't have the most elegant of looks.