Hi all.
I have 3 decks of different sizes that I've neglected for a few years. I want to give them some attention, sand them back (if needed) and give them a clean and oil.
First deck is my main deck.. has had an oil a few years ago but is greyed off in areas that get sun and starting to cup. I've also spilt paint in various spots. About 40sqm.

Another deck is my spa.. never had any oil on it. About 4sqm.

And last deck is my front deck.. was oiled a few years ago. Seems to be getting dirty in the exposed area. About 7sqm.

What I'm wanting to know is if I need to sand these back before oiling? And if so, what sort of effort are we talking? I would hire a sander to do it. Are we talking a day? 2 days? Is it painful work to sand it back?
And would you recommend I clean it first with something?