RE: General building Q, how to fill a low spot
Go to your local hardware store and get a tub of vinyl spackling. Put a big blob of it in your low spot so that it is higher than the rest of the surface. Take a straight and ridged ruler (or any object like it, large spatula) press down the filling and work it to match the other surface. This stuff is extremely light, it sands and feathers just great, paints and so on. Oh, it's tough also! By the way, for the money, you get 20 times what you get in those "other" fillers and epoxies, balloons, that you might get at some "other" store, if you get my drift! You can also use the regular spackling for dry walls. I use them both. Now, if your area is extremely large (doesn't sound like it), then layer with sheets or blocks of balsa, sandable glue, and just sand to match. Then use the above mentioned for filler. It works as well as the light weight Bondo, is lighter, and it's not as hard to sand as epoxies or resins. You can also use this stuff for all around filling (cracks, holes), as well as for making filets and farings! Have fun and make it a nice one!