Weed be gone will kill weeds and not affect established grass, so if you have grass it'll stay there. In PA, you probably have at least some KBG, which will fill in bare spots. But if the spots are big, it'll take a while to fill in. If you have big bare spots, you may want to overseed this fall (maybe late summer, basically wait for the summer heat to break).

Doesn't the seeding require you to wait extended periods of time before you can cut the grass?
What is KBG?

KBG is an abbreviation for Kentucky Bluegrass.

If you're overseeding an existing lawn, what you do is cut it as short as you can first. If you want to keep as much of the existing grass as possible, gradually lower the deck. If you want the existing grass to get stressed and maybe be replaced by the new grass, chop it all at once. It's not going to kill it all, but it will stress it worse and will kill some of it.

Then put your seed down and make sure it stays damp for at least 3-4 weeks, watering several times a day if there's not enough rain. You don't need to water a lot, just enough to get the top wet (maybe 1/2 of the soil at most). All you want is enough to keep the seeds damp. They don't have roots, so watering deeply doesn't help at this point.

You then want to wait as long as possible before mowing. Raise the mower deck to its highest setting and wait until the existing grass is 1.5 times as tall as it will be after cutting (for example, if the deck goes to 4 inches, wait until the grass is 6 inches tall before cutting.

If you've got a fair amount of KBG in the existing lawn, you may not need to overseed. The KBG will spread through specialized roots called rhizomes. Whether the bare spots will fill in depends on how big the bare spots are and how much healthy grass you have.

To keep weeds at bay, cultural practices can help a lot. Mow the grass fairly high (I mow mine at about 4 inches) and water deeply and infrequently.

Thanks for the responses. I recently purchased Scott's Turfbuilder (and a spreader). It said to apply it in the morning (when the dew is still present) or after sprinkling. I am assuming this is so that the granules stick to the leaves of the weeds. I went out this morning and ran my hand through the grass - it was plenty wet. I filled up the spreader and went on my merry way, but I noticed the granules were bouncing off of the weed leaves -they didn't appear to be as wet as the grass. Was this morning's work all for naught? How wet does it really need to be?

I assume the Turfbuilder you bought is the kind with a weed killer in it. Is that right?

The reason they say to use it when the lawn is wet is because in order for it to work, it needs ti stick to the leaves.

I'm not a big fan of the combination products. I think you're better off buying a fertilizer to do the fertilizing and a weed killer to do the weed killing. The reason for this is that when you buy the combination product, you're blanketing the entire lawn instead of only hitting the weeds. So you're wasting a lot of it. Also, the weed killer works best when the weeds are actively growing. You get the best results by fertilizing, then a week or two later (when the weeds are growing like crazy from the extra fertilizer) go around and spot spray the weeds.