I have decking that looks in desperate need of some protector or oil. In the UK it's already quite wintery – temperatures are consistently around 5C (~40F), most days are wet. The instructions for all the protectors and oils that I see say to apply in temperatures over 10C-15C and only in dry conditions. How important is this? Should I assume that doing something is better than doing nothing? Is there any advantage to protector over oil in this situation (I think protectors tend to be water-based, so maybe less sensitive to wet weather?).
How to protect decking when it’s already winter
decking
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Based on questions answered in chat you have a case where the contractor didn't measure the deck properly, and built it too high, could not flash properly under the door frame, and performed a "some genius" move putting a transition strip (meant for interior use) over top of the door frame and the vinyl, with caulk under neath.
Stupid.
Two options: 1 cheap and temporary, 1 expensive and proper.
Cheap: Use proper caulk (props to @tester101)
Option 2 - and the door is old so this isn't actually a terrible idea.
Replace the door. In the process, raise the rough opening of the door frame half an inch or so - however much is necessary to be able to put a proper piece of flashing under the door frame and over the vinyl.
You will probably have to custom order a door to fit your new rough opening, but at least you can seal the door area properly.
And props to @bmitch for pointing out that flashing needs to run the entire width of the deck.
Ordinarily, I'd get 10% extra for a job like you describe. Given the custom setup, I'd seriously consider 15%, and maybe maybe if it wasn't outrageously cost-prohibitive, going for 20%. Splits, knots, and if you live in my world, mis-cuts happen. It would break my heart to have to go back for one more stick...
Plus, if you get way too much, then you'll have the fun of figuring out a project on which to use the scrap. Bench seating? Table? Quadcopter? (Okay, maybe not the last...)
Best Answer
The temperature range is important because it dictates the drying time and should be followed pretty close. The dryness factor is critical. Wood absorbs water. When you set out to protect your deck you want the deck to absorb the sealer. If the deck is already wet, then the sealer won't sink into the wood and you've wasted your money. The deck must be dry so it absorbs the maximum amount of sealer. This applies to water based sealers too.
If you feel the need to do something now, You can get a water proof tarp and cover your deck and then apply the sealers when the deck is dried out. Make sure to allow for air flow while the tarp is on the deck. Then keep the tarp on the deck until the sealer dries, still allowing for air flow. Otherwise, wait until spring. Good luck.