You don't mention whether these will be for outside or inside seating, so below assumes outside.
If you don't mind them eventually turning a silver gray eventually (especially the cedar), teak oil works very well. Rosewood oil, a few brands I'm aware of available here, have more added UV inhibitors and works quite nice for keeping the color longer.
If the benches are lightly scrubed down, allowed to dry and recoated for the first couple years will hold color longest. Though this may fall into the unwanted maintanance you are trying to avoid.
When I've done these two species for customers (well and teak), I've offered to either come back in a season and do it for a very low fee or offered to give them a small container for them to do it themselves. They've all chosen to do it themselves. One person's outdoor furniture has remained very nice from them doing it.
I also recommend the client as regards outdoor furniture, to cover or store them out of the weather during extended periods of non-use or the winter months.
Thanks for reply. The western cedar is cladding which is fixed onto the side of a outside 600mm high retaining concrete wall. The douglus fir sits on top of it to provide a seating area. I need the cedar to retain it colour for at least three years. What colour does douglas fir go with age?.
Simon, both will eventually turn gray. The cedar will be a nice silvery gray, the doug fir will become, well, kinda ugly gray.
The doug fir will also have a tendancy to attract mold spores if left unsealed and get some blotchy black/green patches to it.
For the cedar, the best I can recommend is to let several coats of the rosewood oil soak in and dry thoroughly between coats (about a day each) and then use a spar varnish (I think it's called marine varnish there?). A few good coats of it does wonders because of its UV protection.
The problem is the doug fir. Spar varnish remains a very flexible finish. This can become tacky/sticky in direct sun. While it will help the color, people sitting on it may stick to it on a warm day!
Again, the rosewood oil will provide some good UV protection for the doug fir, but the film finish is the issue. I would make up a sample board of doug fir and put a few coats of spar varnish on it. Let it fully cure for about a week out of the weather. Then place it outside on a hot day and see if it is acceptable.
I would also contact Bob Smalser, he's a member here though he doesn't visit often. A PM might alert him to a question. He might know if you can alter the properties of the spar varnish to provide a harder finish (Terry Smart of Chestnut Products may also know come to think of it) and still provide UV protection.
Hope you don't mind me asking - where did you get your timber from? I posted on the woodworking site re WRC sources and am interested in quality response rather than just the best price I can get from a yard.
that was very help full. Just had a funny picture in my mind of the mother in law stuck by the seat of her pants to the douglas fir on top of garden wall :twisted: ,hmm might come in handy keeping the birds away. .
Picked the douglas fir up from covers in chichester normally use goodwillys but they had none in stock. As to the quality the boards they were free from twists and shakes and the grain pattern is beautifull. Went through the stack myself was charge for 4.2m lenghs but they were really about 4.3m which i was happy about.