My first Flow Hive is on the post office right now, I have never owned this kind of hive before.
If I treat it with cedar oil, how often do you think I need to reapply that to keep the hive nice and functional in a pretty wet and cold climate?
Anything I should know about this hive that you have allready gained experience of here on the forum?
Welcome to the forum where you find really nice folks happy to pass on good advice and tips. You will find lots of information if you click on the magnifying glass on the top right of your screen.
There is lots there about how to better use a Flow Hive to prevent problems like flooding when your extracting the honey.
It seems most use Tung Oil on their Cedar hives, I guess mold will be an issue in your climate and there is no given time for re-coating, Maybe better to go by the appearance of the hive as time goes by.
Cheers
Hiya Ulf, welcome 
I first coated my cedar Flow super and brood box with tung oil, which looked very nice but this finish degraded and allowed mildew to start near the finger joints after two seasons. Being in a climate with lots of summer heat and humidity and freezing winters I decided to sand the boxes down and recoat with an exterior grade water-based polyurethane. As direct sun made the wooden roof quite hot, I decided to paint it a light yellow color, which was a good move for my setup. It seems a shame to paint cedar, but it made sense under the circumstances. I did leave the gable ends unpainted/coated in poly to keep the wood grain look. Third year on, the poly is still holding up!
Hi. I’ve just got my first cedar flow hive and not sure whether to give it a coat of protection, tung oil?/polyurethane? We have hot summers here and cool winters. Do I need to do anything to protect the roof?
Cheers
You need to treat a cedar hive before you install any bees. A cedar hive will go grey fairly quickly because of the sunlight, but in wet weather it will be a big problem with mold. Tung oil on the boxes to keep the natural timber look but it will be an annual event to re-coat them. Polyurethane is another option that should be more mold resistant. I would advise painting the roof white to help the hive from being too hot in your Summers. A couple of coats of the tung oil and paint is way better than a single coat.
Cheers
Anyone ever try Spar Urethane? I’m wary since the assembly instructions say low VOC, water based, etc., but Spar urethane is such an excellent outdoor water proofer and i live in hot swamp New Orleans. I have the cedar hive for termite protection and was hoping to leave the natural finish. Polyurethane would last a few months before flaking and turning grey
I thought about it, and our front door is coated with it (by a professional). Our current humidity is 65%, in summer monsoon weather, it can get up to 80%+, so that does a number on the wood and the varnish. However, in SoCal the UV also is lethal to coatings. I haven’t tried the spar urethane products on our hives, but they work very well on our house! 
One more thought. If you let it off-gas for a couple of months, I don’t see a problem. Less than 4 weeks, the bees may not like it. You could get a temporary cheap pine hive to house them in, while you finish their palace though!