Thanks for the lovely picture, kbeitz. Diesel oil and tar are non-toxic? Maybe compared to the arsenate-treated and worse, creosote-treated stuff. An oil-tar mix would be a good herbicide, similar in action to the old "carrot oil" used to kill roadside and railbed weeds, including some hard-to-kill perennials. Cedar is naturally rot-resistant but weathers irregularly. Most treatments will darken the older wood. If you like the color already you might try a water-proofing treatment applied several times over the course of a week or so. You can also use boiled linseed oil thinned with perhaps paint thinner (is there a less toxic alternative?) and brushed or sprayed liberally. The surface will darken. If you ever want to darken/"age" new redwood and some cedarwood, make a "paint" of baking soda and water and brush on for a darkened look. I do that on redwood benches and fencing for that instant weathered look. The dark colors fade with time to true aged ones.

I just posted about what I used on my red cedar siding in the Deck Sealer thread. It won't leave your siding looking like it does now (it's not a clear coat), but it looks pretty good. Here is a link that might be useful: Deck Sealer Thread

When on cape cod in the mid eighties, a bleach solution was used to "blanch" the shingle, rid mold, and get it closer to looking like a newer applied shingle. Sorry i dont know what the mix ratio that was used, but you might ask siders or painters in your area if the technique is still used and the mix ratio if it is.

I must say, the aged look of the shingles is quite beautiful to me. Sounds like (from the above) that doing your spot repairs as needed could be the way to go, both esthetically and financially. May I ask pros for more detail about how you would prep either for clear finish or paint if thats the route someone wanted to go? If wood is dark do you assume that its mildew (prob use deck clearner type product?) or is it just some natural aging (patina) of the wood that could take a clear finish over it? Just curious - dont have a shingle house at the moment but almost did once. If the damage is limited conceivably OP could use an epoxy-base d wood hardener + wood fill product on it? Generally my experience with 100 yr old wood is :-) and not :-( This is likely still old growth timber and as such was hard as nails very unlike the farmed wood of today (and still is, if its not too rotted) Was so shocked when I got some cedar trim milled for my old porch at how soft and splintery it was - nothing like the existing wood. And I would reiterate about trying to find someone local who is old house friendly and has wood restoration in his/her repertoire - either to do the actual work or you pay them for a consultation. Your plastic guys - siding, windows, etc. - have their own business model and ways of doing things so you really cant expect them to be knowledgeable in other methods. Its fine with me if thats what they want to do, but where I have a problem is when they dont recognize their limitations and make these pronouncements about how the old siding or windows have to go .... like its some kind of ulitimate truth. Consequently, a lot of people dont look any further and the end result is ... at least in my neck of the woods.... a lot of plastic-clad houses.