Hi, first time poster, first time hardwood floor refinisher here.

I am sanding/refinishing 600 sq ft of 100 yr old maple. It is
3 bedrooms and a hallway. The closets in the 3 bedrooms are pine. I am not staining. My plan was to do 2 coats of Bona Classic Seal and 2 coats of Bona Traffic HD extra matte.

I have 2 coats of sealer down with about half a gallon left on the third jug. First coat was applied in the afternoon and allowed to dry overnight. Then second coat applied that afternoon and allowed to dry overnight. I was considering, rather than throwing it away, using the last half gallon as a third coat over the hallway and in the pine closets where the layers don't seem to have as much build. Then i would apply two coats of traffic after that. My question is: will adding a third coat of sealer in hallway/closets be harmful?
I'm concerned having too much product down could interfere with curing, but i also like the idea of having extra build on the pine and in the high traffic area.

Thanks for any advise.

You ought to follow the directions on the bottle for each finish product. If you bought enough finish for your square footage and you did not use all of it as recommended, then you can add the extra coat. Usually people apply too thin of a coat. If you got extra finish because it comes in gallons and you applied at the recommended coverage and have some left over then save it or give it away. The problem that you can have with seal coating material is that it is made to be harder than the finish. This helps when the grain raise is being buffed off with a screen or pad. If you have too thick of a coating of sealer you may have more chance of the finish chipping when it is stressed like from something dropping on it.
The finish on the other hand is made to be flexible, so it will not chip as easily. It will stretch to follow a dent in the surface. You can add extra coats of finish for a thicker wear layer. The surface will scuff and then you can see scratches over time and the surface will dull but a thicker coating will protect the surface from abrasion.
A thicker film will take a little longer to fully cure, but you can use stocking feet for a little time to prevent scuffing. Place your furniture and don't drag anything across the floor, ever. Use floor protectors for chairs. Wheels may leave dents and crush grit into the surface.