I'm a total novice but linseed oil to me is more of a finish for interior or food grade stuff. I'm still learning the difference between stains, finishes, etc. My wood worker neighbor tried explaining it to me and since there is some overlap it can get confusing. I did some googling and found this https://bearcreeklumber.com/general_info/guide-to-finishing-western-red-cedar.html#:~:text=In general, finishes for cedar,; and (4) oils. Then I continued reading more, and more.... It's actually a really good bit of info since your gazebo is cedar and that's all about cedar. There's a section about decks that may be helpful for you. Someone else might be able to pick out a specific product for you but that article might help explain it well enough to pick out a product type.
The greying comes more from the UV hitting it than anything... the oiling will keep it from drying out and checking badly, but won't stop the UV damage. Some oils might even speed up the process. Kind of like how you burn faster if you lather yourself up with baby oil instead of slathering on the sunscreen. I've personally used Epifanes spar varnish due to the very high and effective UV protection it has due to the additives they put in specifically for that... it's like embedded sunscreen in your finish. It seems to be the go-to finish for high end wood boats, like those classic Chris Crafts. https://www.epifanes.com/page/clear-finishes It is pretty expensive, though... I figured the cost was worth it for a table, but not sure I'd spend it on a gazebo. They have a few different finishes... gloss, matte, etc. I used the matte finish on my cedar table I made for my deck, and it sits out in the elements year round (snow, rain, sun) and still looks good 5(6?) years later. I expect that I'll probably lightly sand and refinish it this year, but the underlying wood has really kept its colour to that deep reddish cedar as opposed to greying out (other than where a bird/squirrel ate through and edge or something, where you can see the wood greying out as a result). I'll post a pic in a bit of what it looks like now.
Here are some pics I just took. There are some tree buds/etc all over it, and it's been raining, but you get an idea of the finish after zero protection for 5-6 years of not covering it in the winter or rain. It gets ugly in the Spring, but a quick soap/water wash with a brush cleans it up pretty well. I think the wood colouring is lasting quite well, especially when compared to the rest of the deck, etc. And to be clear, these were Home Depot 4x4" cedar fence posts that I glued up to make the table, so nothing fancy. Quite the opposite of fancy.
Here’s a closeup of where you can see some edge damage that is maybe 2 years old. Quite interesting just how quickly the wood ages without that protection.
And now I'm all mixed up. Read a lot of places that said to never finish cedar with anything that hardens. Shellac, varnish, lacquer etc because eventually it will crack and chip off. Thought I settled on a product from Cabot but a lot of online reviews said it turns their wood yellow so I called Cabot tech support and the gal said I need to let the cedar age for ~6 months, pressure wash the gray off and then seal it. (WTF?) I might try the linseed oil that 'wildered suggested. Pretty sure I have some and will try it on the backside of a post.
The varnish that I used is quite flexible. It's flexible and has UV protection. Again, it's specially formulated for marine applications.
If you scroll up on that article I linked, it actually talks about NOT letting the wood age. I think letting it age is older advice. However, this is me repeating shit people have recommended. I will ask my woodworker friend what he would use for the application and maybe he could rec a product for you. I think linseed oil outdoors is going to magnify the UV damage like Nett suggested. It helps keep wood pliable and moisturized but not protected from UV. If preserving the color (the greying is environmental dirt/mildews and UV damage) is your goal I think you need a different product.
Depends on your distributor... I found a place in Toronto that it worked out to be about $80 a litre. I think it was easily twice the cost of a Home Depot stocked spar varnish that claimed to do the same thing (but not as well based on tests I saw). The Wood Whisperer turned me onto it in a video years ago where he was looking for some sort of outside trellis/gate finish. There might be new products on the market now, like from Total Boat (that has blown up in popularity over the past few years), but I find that it's mostly the marine applications that were performing well.
To answer @bewildered 's rep, that damage is due to a combination of squirrels and birds eating/clawing at it for some reason. Some birdseed got dumped on the table one winter and the birds flocked to it and were pecking the spill right from the table (maybe 2" of snow cover at the time), and it caused some damage to the surface (cedar is soft, yo).
How many coats did you do on that table? I've seen boaters go as high as 12 coats on their teak, sanding between coats, thinning the epifanes some also
4 coats. First 2 were the gloss that was thinned 20% then 10% in order to flow and soak into the grain. 3rd coat was then straight gloss. 4th coat was then the finish coat, straight matte finish. The matte finish is more expensive, and you only need it on the final coat to get the effect, so you can use the normal clear/gloss as the base.
I posted pics of this years ago (can't find them off hand), but the biggest mistake I made was not filling in all the knot holes properly and completely. Epifanes offgasses a LOT (which is why you need to wait 24 hours before applying a new coat), but if it pools up inside a knot, etc, then it will offgas like a BITCH. It was so stretchy that it offgassed so much that I had huge blisters forming on the table top as the stuff was still curing in the deep pools inside the knots. It was like someone was blowing bubbles from underneath. It was nuts. When that happened I had to sand it all down a bit (not to bare wood, just to get imperfections out of the surface), fill the knots with epoxy and/or CA glue (that I tinted a dark brown), then did the new Epifanes finish on top of that.
Looking at the dates, I finished that table in 2017. Seeing as that's 6 years ago, I'm pretty damn happy with how it's literally weathered the storm.
Looking back through the posts, I totally forgot some of what I did. Apparently I used a bunch of Tung oil initially on the wood to help oil it up and maintain it's condition. After it all dried up well, I then finished it with the Epifanes spar varnish. The Woodworking Thread
Thinking spar is the way to go based off Nett's results and found this page https://woodworkly.com/spar-urethane-on-cedar/.
Good luck dude... hopefully it works out well for you. The only downside is that I imagine redoing a gazebo would be way more work than the simple table top that I ended up fixing. I hope you post some pics of the process.
Well, the Epifanes spar varnish is out. I called them and they were extremely helpful but the dude basically said it would be a shit ton of work and upkeep for a gazebo with all the different posts, boards, gables, runners etc. Eight coats at a minimum and more coats every ~ 2 years. If I lax and it cracks, I have to sand back down to bare wood and start building it back up. I specifically asked about Nett's table lasting so long but when he heard it was in the frozen tundra and in a pretty shaded back yard, he said my experience would be much different as it gets ~8 hours a day of sunlight on it. I asked him about boiled linseed and even though they don't carry anything like that, he said it's not bad but plan on a much more, although easier, upkeep. He recommended this stuff: https://www.epifanes.com/product/00/tob-500/Teak-O-Bello-500-ml I already have all the BLO, so I'll do that and get the Teak-O-Bello later if I don't like it or if it fails.