Can You Or Can't You Sand Down Out Side Wood? Have I Been Given Bad Advice.

[flowernm66]

Hey.
Sorry, this may be a bit of an odd question. 
I am building a planter and a small wooden shelter outside for the bins.
I will admit I am new to DIY and wood types and treatments. So i popped into the only DIY shop in my area (Wickes) and I was asking about wood treatment. I mentioned that I planned to sand down their rough garden posts to a smooth finish for the shelter.

The guy who looked about 18 instantly got a quite obnoxious look on his face and said in a very patranising tone "You really are new to the game arent you? You NEVER sand down sawn timber that is made to go outside." When I asked why not? He seemed rather stumped and mumbled something about "The paint doesn't adhear to it as well and you sand off the treatment". 

This to me sounds completly incorrect and more like he didn't know anything about wood (Or would that be me?) I have looked online and can't see a thing about it. Is there ANY truth to this?
 

[flowernm66]

Side note: The post i was going to sand down had been pressure treated. I was also going to use a wood pretector over it anyway. 

[douglas9401]

It would be very poorly 'treated' wood if you could simply sand off the treatment.

Apart from a complete waste of time, why bother when finished treated wood is available?

[flowernm66]

@douglas. 
Its about £6 cheaper than the sawn wood AND gives me a reason to use my new sander.... also, I'm tight lol 

[flowernm66]

*More expensive. Sorry. 

[Maydup]

He is young and very poor  communication skills - but right in a way.

We have made fences and planters and so on and tended to use treated sawn wood as the finish soaks in better. If you sand it down you find the finish sort of slips off and takes many more coats to cover.

Obviously if you want a very smooth surface then sand it by all means. It's hard work but not against the rules. 

[The Builder]

Flower, you're right .  The fella is talking out of his tabard.

Of course you can sand any timber down. He's confusing sanding with planing. I think that's what Maydup was referring to. Planing timber sort of 'seals' the grain. It changes it from an open-textured grain to a closed surface. 

Planed timber is available treated, but it's pressure-treated. No such treatment can be 'sanded off'.

Use a coarse belt/disc for the sanding or it'll keep clogging up.

Yes he was 'half right'. I think there was just a little confusion.