To review: Our Friend the Sun degrades unprotected wood by destroying its lignin, which is the stuff that stiffens its backbone. Without its lignin, a piece of wood isn't much good; as I observed earlier, it has a consistency similar to stale cornbread, which only goes to figure, since most of what is left of wood once the lignin is gone is carbohydrate.
When this happens to structural wood, it's best to replace it if possible, as I did with some of the sash on the second floor [see Journal]. Nevertheless, wood in this condition is still salvageable, and with a Victorian, making do with what you have is often the simplest and most economical option, because you're not likely to find a suitable replacement at Home Depot. Our contractors had to have everything they used milled to order, and while they left a lot of this wood behind, they left none of what I need on the south side. So rehabilitation of what's there is the only option.
Rehabilitating lignin-deprived wood is actually quite simple. All you do is replace the lignin with epoxy. I use Abatron's LiquidWood, which is specifically formulated for this sort of use; it has the consistency of corn syrup, and lignin-deprived wood soaks it up like stale cornbread soaks up milk.
The next day, when the epoxy has cured, what you have is zombie wood, once dead but now re-animated to do your bidding. The only difference is that zombie wood is not out for brains; it only seeks to return to its former usefulness.
Of course, a problem remains: it looks like zombie wood. More prosaically, it looks like it did before, but shinier. In truth, it is now a piece of fiber-filled plastic resin, more durable and resilient than wood; it is certainly restored structurally, but it still needs to be restored cosmetically.
For that, I use another two-part Abatron epoxy, WoodEpox. It's a bit of a pain to use, because it's rather messy to mix together, and you can't mix together more than a few ounces at once, so you have to keep stopping to mix more. On the other hand, it is very easy to work with in terms of applying it; it comes off the putty knife easily, can be feathered out very thinly, and it will not shrink or sag. It also sticks very well, especially to cured LiquidWood, is extremely strong, and is easily sanded. Because it hardens via reaction and not evaporation, it can be applied in any thickness. Simply put, it's neato.
So that's my next step, restoring the original profile of the belt course trim using WoodEpox and my wits. And a putty knife.