Sanding treated wood advice

johnny2007 · 18 May 2020

Hi, just to show my brother who doesn't believe me, can you please advise on the following:
He's refurbishing an old bench and replacing the slats.
Budget didn't allow for hard wood, so he bought 3x1 kiln dried treated wood.
He cut the correct length, sliced 4 of the slats that needed to be 2x1 and dipped all cut ends and the sliced slats in wood preserver for 3 hours.
Then he sanded them smooth.
The wood was not very rough, he showed me an off cut, so he used a 120 grit disc on the orbital sander.
The finished product is smooth and still greenish.
I told him to go straight with 3 coats of exterior woodstain, but he wants to dip them again in preserver before doing so.
IMO dipping them again is overkill and also will possibly spoil the smooth finish, so requiring further sanding.
What do you think?
Dip again, re-sand and stain or just go with the exterior woodstain as i suggested?
Thanks

opps · 18 May 2020

Is he using a waterbased preservative? I have never noticed grain swell with a solvent based version.

johnny2007 · 18 May 2020

Sika wood preserver.
Got to check...

opps · 18 May 2020

It's waterbased,hence grain swell.

Burnerman · 18 May 2020

I think all will be well with what’s been done already.....just paint the woodstain heavily on the end grain before bolting the slats back on.
John

johnny2007 · 18 May 2020

Surely the end grain will be fine as they were dipped for 3 hours.
Usually I dip them for 15 minutes.
He's concerned about the rest of the wood, especially the sliced lengths.
I tried to explain that timber is dipped in massive drums and pressure treated, so on a 3x1 the solution penetrates basically to the core.
He doesn't believe it.

opps · 18 May 2020

johnny2007 said:
I tried to explain that timber is dipped in massive drums and pressure treated, so on a 3x1 the solution penetrates basically to the core.
He doesn't believe it.

If it penetrates to the core, why would it need end grain sealing?

justbigboned · 18 May 2020

120 is pretty aggressive and on an orbital could remove lots of material very quickly. It may not be necessary to re-treat but it depends on how much sanding was done. If it was sanded back from rough sawn, then I would re-treat, personally.

If he does treat it again with the Silka stuff the grain may swell, but he can then simply hand sand it back, starting at 180 and working through to 240 or even 320. That should give a really smooth finish without removing too much material.

opps · 18 May 2020

The following says that the pressure treatment penetrates "a few millimetres".

https://www.wheeliebinstoragedirect.co.uk/tanalised-timber-faqs/

johnny2007 · 18 May 2020

He hasn't taken more than half a mm off.
I've seen the initial finish and it wasn't that rough.
In fact, i managed to get it smooth with 180 grit paper by hand in no time as a test, just now.
And i think he's used finer paper than 120 as the result is pretty smooth.
I've now brushed a coat of preserver on half a plank he brought here to see if it swells.
In the mean time he's having lunch and drinking my beer?!?!

opps · 18 May 2020

johnny2007 said:
He hasn't taken more than half a mm off.

I think you misunderstood my post. Apologies if I wasn't clear.

My link countered your suggestion that vacuum pressure treating permeates all the way to the centre of the timber. If the link is correct it only goes a few mm deep.

I have no idea how deep waterbased preservative penetrates and I would have imagined that solvent based penetrates further but I could be wrong.

That said, given that none of the timber is in direct contact with water, I would agree that more coat of preservative is possibly overkill but I don't think it will harm it.

johnny2007 · 18 May 2020

Well, as he drank more than I wanted him to, I offered to drive his car home (half a mile).
So while he was brushing the wood with preserver as suggested, I drank HIS beer whilst sunbathing on HIS sunlounger.
I think I've done well, even if he said I was wrong.
Done now, thanks for the input.

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Sanding treated wood advice

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