If you do decide to use any fairing compound, do it properly. Forget Bondo, it is junk (mostly polyester resin, which doesn't work well on wooden boats, mixed with talc). Use either a good epoxy-based, premixed marine fairing compound, or mix your own with epoxy resin and microballoon-based fillers. How hard or soft it is and how much the filler sands like cedar (or does not) depends on the mixture used and is very adjustable if mixing your own. There is no reason to assume that your filler will be harder than the wood, unless you make it that way.
Good fairing fillers usually contain a lot of filler powder and a very small amount of resin. You want to do a bit of testing before you start putting it on the boat, and while you do, keep very accurate track of just how much resin and how much filler the mix contains. You want to be sure that it will not sag, run or puddle-out throughout the entire hardening process and you want to be able to accurately duplicate that mixture ratio through any number of small batches while working. If it does sag or pool, add more filler powder to the mixture ratio and try another test. The mix is right when there is no sagging at all, and you will likely be surprised at how filler-rich the proper mixture will be.
Once you have your ratio figured out, the best way to apply it to a hull is with a notched trowel or spreader, and you will likely have to make it yourself. A flat rectangle of stiff metal or plastic maybe 4" tall and 6"-8" long works well. Here is the trick.... into the bottom edge you cut square notches, typically maybe 1/8"- 3/16" deep, 1/8" wide and spaced about 1/4" apart, for the full length of that edge. It ends up looking like some sort of coarse saw-toothed edge (real, triangular saw teeth don't seem to work well from experiments I tried).
You spread the filler over the surface using the notched edge and enough pressure that as a result, you are basically combing a very neat series of parallel square ridges over the area with uniform (pretty much bare) spaces between them. We then let them harden, and since we used the right mix ratio, the ridges stay crisp and don't sag. Then we get out the longboard or long sanding gizmo and start sanding the tops of the ridges down. Usually you work diagonally when longboarding a hull for the fairest finished surface. You sand a lot off - very nearly down to the wood in the high spots, because you are fairing the shape, not trying to add skin thickness.
Since you are only sanding ridges, not solid fill, there is a lot less sanding needed to get the filled areas trued-in and fair, so you save a lot of work and effort. Once the ridge areas are fair, you mix up more filler, use a plain spreader riding along on the tops of the ridges and fill the spaces between the remaining ridges. Let that harden and give the whole area a quick final sanding. Some very large boats have had their entire hulls faired this way, but it also works well on smaller boats or portions of them. It can be a bit more work than just slathering a big blob of fill on and trying to sand it fair, but it's one of the best ways to generate filled spots which are truly fair and don't show through your final finish as obvious fills.
