I've always trusted Cabot, they have a clear wood protector that has a good UV component. Anything you apply will need to be recoated every 2 years

First thing to understand is that anything offering UV protection will have pigment. Usually a light yellow or amber is the lightest color you'll be able to find. Pigment = protection.

I am a big fan of Sikkens Cetol 1 RE and 2,3 RE system. The (lack of) color options can be a deal breaker for some who want something with less color. Remember though that pigment = protection.

Also the Sikkens lasts longer between reapplication. I've seen commercial projects where it was used on siding and is only reapplied every 7 years or so. YMMV depending on exposure to the elements.

Thanks all. There seems to be such mixed reviews of Sikkens.

7 years between reapplication is great! @Everyday Intern, do you have experience with Sikkens Cetol SRD RE? Have you seen how it would age on wood? Does Sikkens products flake, in general? The good thing is that the wood is all north exposure and it's in a dense urban site so it's not like there is intense sun all the time.

I'm accepting that there will be pigment in the product. Just a matter of finding the right colour for the thermally treated Ash. Cabot only has the yellow stain colour (even with their transparent product). 

Any other comments would be appreciated.  

I don't have any experience with Cetol SRD RE unfortunately. I haven't noticed any flaking of the Cetol 123 system. Northern exposure will help, how are your overhangs for rain protection? The application I'm referencing sees moderate sun (southern and western exposure, but shaded by trees and buildings most of the day) and has very large overhangs and so it sees very little rain.

I've heard from one of the 'old timers' in the firm that he's tried just about everything for wood finishes, and Cetol 123 is his go-to product ... (paraphrasing) nothing better out there for longevity, maintenance, and protection. This 'old timer' worked for a firm that did/does a lot of exterior wood on their projects. 

7 years maybe, depending on climate, quality of application, etc. If the thermal Ash performs as they say it will you shouldn't need any protection. Save some dough and let it weather, you can always coat it later.

Weathered wood is beautiful if part of the design. If the building is meant to have regular maintenance and that doesn't happen, it's less appealing.

Beautifully weathered wood is also a product of climate and exposure conditions...

Love soft gray weathered wood, but it depends....Sea Ranch is a prime example, starts looking like Manson's Spahn Movie Ranch, much of the original stuff is facing replacement.

Charles Moore 1969

Even with a product like the Sikkens semi-transparent stain (which is the best performing out there), you will need to re-apply regularly to keep everything looking good (the most exposed side will determine the interval of your application). That sikkens product has a ton of colors to choose from that can provide the silvered look without the localized "staining" that naturally occurs with letting wood silver on its own. By protecting the wood of your facade, you are extending the life of your cladding. I'm always surprised how quickly wood can get hammered in the sun/salt/water etc. Your siding provider can likely get your materials prestained so that your framer/sider only needs to do end touch-ups.

 

Edit to add: 7yrs is ambitious, plan on three (but maybe your climate is milder than I'm guessing)

My parents get exceptional mileage out of their Sikkens application.  Much more than 3-years.  Maybe more than 7, but maybe I'm not paying as much attention to how often they do it.  Their exposure to all-day direct sun is limited.

Climate here is more than just mild unfortunately. I'm in Toronto, Canada so we get both extremes: very hot and very cold. The good thing is that the site doesn't have harsh sun. It's north exposure and it's a dense urban site with mature trees and neighbouring houses very closely situated. We also have a garage facing the rear of the house. Anyway, my point is that it's not in an open field where sun is beating down all day. 

I'm leaning towards Sikkens. I did a finish samples on separate planks of the wood cladding. Some has Cetol 1 first coat, and Cetol 23 second coat. Another plank has Cetol SRD. And then there is a control sample that will be left untreated to compare. I'm not sure if I want to leave it to patina to silver grey. I'm afraid it'll look blotchy and it will be no turning back once it silvers...