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External iroko - Osmo UV oil or Sikkens Cetol Filter 7 Plus

Help with choosing the right coloured milkpaint to slather all over your new project.

External iroko - Osmo UV oil or Sikkens Cetol Filter 7 Plus

Postby RogerM » 12 Oct 2014, 10:38

Nope - I haven't a clue either, and I've had enough conflicting advice to convince me we need a test, so I've set one up.

This is another thread from the old WH site that I've been asked to repost. Available elsewhere! This test was started in March 2012.

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I've used an offcut of pale iroko, and from the left I have 3 coats of Osmo UV Oil, then one coat of Sikkens Cetol HLS Plus light oak stain covered by 3 coats of UV oil, and finally on the right one coat of Sikkens Cetol HLS Plus light oak stain followed by 3 coats of Sikkens Cetol Filter 7 plus light oak. The test piece is out in the open facing due south in full sun so we'll see what happens. I'll update the thread at approx 6 month intervals, or when I remember or when asked.
Last edited by RogerM on 12 Oct 2014, 10:46, edited 1 time in total.
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Re: External iroko - Osmo UV oil or Sikkens Cetol Filter 7

Postby RogerM » 12 Oct 2014, 10:40

Reviewed October 2012.

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All 3 seem to be standing up well. Predictably, the wood has darkened so that the difference between the far left and the centre panel is less marked, but all seems fine so far. The test piece has been in full sun and open to the weather.
Last edited by RogerM on 12 Oct 2014, 11:24, edited 1 time in total.
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Re: External iroko - Osmo UV oil or Sikkens Cetol Filter 7

Postby RogerM » 12 Oct 2014, 10:41

Reviewed June 2013.

Latest update is that they are all standing up well. However, as you can see, there is very little difference between the UV oil over the stain compared with the UV oil on its own. There is a narrow line of grey along the top edge which may be due to UV getting to the wood from both sides near the edge. The Sikkens is still looking solid and if this were joinery I wouldn't feel the need to repaint yet. The position remains facing due south in full sun

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Re: External iroko - Osmo UV oil or Sikkens Cetol Filter 7

Postby RogerM » 12 Oct 2014, 10:43

Reviewed Jan 2014.

Getting a bit scruffier! Now nearly 2 years old - including 2 summers.

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The section of the left which is just 3 coats of UV oil is showing severe breakdown along the top of the specimen, and it's starting to show signs of grey along the front as well.

The centre section, which is one coat of Sikkens Cetol HLS Plus light oak stain covered by 3 coats of UV oil is slightly better, which seems to indicate that a bit of pigment does help, which is hardly surprising given that sunglasses help as well.

Finally the one on the right has one coat of Sikkens Cetol HLS Plus light oak stain followed by 3 coats of Sikkens Cetol Filter 7 Plus (light oak), and this is still holding up well. There is slight greying along the front edge, but the front face and top is generally looking good. I reckon that if I had chamfered the edge, or rounded it slightly that would have held up as well.

Ignore the white spots - back in the summer some idiot :whistle: sprayed his kitchen doors nearby and the overspray drifted over the test piece.

Incidently, the back of the specimen which was treated in exactly the same way and is in the shade, is pristine, and it is slightly disappointing that a product that is marketed specifically as UV Protection has come out the worst. Whilst Osmo would probably claim with some justification that this is not a scientifically controlled test, and one specimen is not enough, this IS a real world test based on how people actually use the product in real life.
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Re: External iroko - Osmo UV oil or Sikkens Cetol Filter 7

Postby RogerM » 12 Oct 2014, 10:44

Reviewed Oct 2014.

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This piece has been in full sun and salty wind ( we live on the coast) for 2 1/2 years now. This is the end of the 3rd summer. The left side (3 coats of Osmo UV Oil) is faring worst, particularly the top edge where not only has the wood discoloured, but the finish is peeling. The centre panel (one coat of Sikkens Cetol HLS Plus light oak stain covered by 3 coats of UV oil) seems to be holding up a little better. By far the best though is the right panel (one coat of Sikkens Cetol HLS Plus light oak stain followed by 3 coats of Sikkens Cetol Filter 7 plus light oak). Whilst this is beginning to show signs that it is weathering, I think it will probably last another summer before it would need repainting if it were a piece of joinery. However, I'll just leave it to continue weathering to see how it lasts.
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Re: External iroko - Osmo UV oil or Sikkens Cetol Filter 7

Postby Rod » 12 Oct 2014, 11:14

Thanks Roger - that confirms my experience and have been using the Sikkens system on my external woodwork for several years now.
I tend to retouch the south side every 4yrs but the north facing ones seem to last for ever?

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Re: External iroko - Osmo UV oil or Sikkens Cetol Filter 7

Postby Andyp » 12 Oct 2014, 15:37

Thanks Roger,

That confirms my experience with Osmo too.
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Re: External iroko - Osmo UV oil or Sikkens Cetol Filter 7

Postby Mike G » 14 Oct 2014, 19:35

Not mine, though, and it is possible that this is because the OSMO product is, from memory, not recommended for use on horizontal surfaces. It could well be that the top edge breaking down like that is due to water ingress, frost, or the like. Where used in accordance with the manufacturer's instructions, I have had great results from the Osmo UV product, including on the outside of commercial and public buildings. What I liked most was the original colour, and the fact that it barely changed.
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Re: External iroko - Osmo UV oil or Sikkens Cetol Filter 7

Postby Andyp » 14 Oct 2014, 20:25

Your are right Mike but I made the mistake of trying to use Osmo on an outside table. The horizontal surfaces, after less than one year, were just like Roger's example.
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Re: External iroko - Osmo UV oil or Sikkens Cetol Filter 7

Postby RogerM » 21 Oct 2014, 14:08

Mike G wrote:Not mine, though, and it is possible that this is because the OSMO product is, from memory, not recommended for use on horizontal surfaces. It could well be that the top edge breaking down like that is due to water ingress, frost, or the like. Where used in accordance with the manufacturer's instructions, I have had great results from the Osmo UV product, including on the outside of commercial and public buildings. What I liked most was the original colour, and the fact that it barely changed.


You're right Mike in saying that UV oil is not recommended for horizontal surfaces. In this case the sample is vertical, but with a horizontal edge. I think most external surfaces, even vertical ones, will have a horizontal component somewhere. I was very much aware that to be a fair test, the product needed to be applied in accordance with the manufacturers instructions, and it was. However, even the vertical surface is showing signs of weathering.

All this proves of course is that UV oil was not the best way of protecting this particular piece of iroko, and nothing else. It is however a "real world" example, and all i'm doing is showing what I've found. Others may get different results with different species of timber, different locations, and with the timber presenting a different aspect to the weather and sun.
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Re: External iroko - Osmo UV oil or Sikkens Cetol Filter 7

Postby RogerM » 16 Mar 2016, 11:31

Can't believe it's 17 months since I reviewed this.

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This test piece has now been exposed to the weather for exactly 4 years. The left side - 3 coats of Osmo UV Oil has continued to deteriorate and there is little sign of any protection left. The centre panel (one coat of Sikkens Cetol HLS Plus light oak stain covered by 3 coats of UV oil) seems to be holding up a little better, which demonstrates that you do get some extra UV protection from pigment. The best though is the right panel (one coat of Sikkens Cetol HLS Plus light oak stain followed by 3 coats of Sikkens Cetol Filter 7 plus light oak). The vertical part is remarkably good, although the top/horizontal edge is showing signs of weather. After 4 years in a salty environment facing south in a particularly sunny spot, this is hardly surprising and if it was external joinery I would be redoing it this year. It case there is any doubt as to whether it is the sun that is doing the damage, I've also posted a pic of the back of the piece which is in permanent bright shade, and none of the finishes have been affected to any noticeable extent.

This is the same Sikkens finish on a sapele post box which gets full sun for half the day, applied about a year ago. No sign of any degradation yet.

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Re: External iroko - Osmo UV oil or Sikkens Cetol Filter 7

Postby Rod » 16 Mar 2016, 14:03

That's nice to know - I've been using the Sikkens system for many years on my Windows and doors.

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Re: External iroko - Osmo UV oil or Sikkens Cetol Filter 7

Postby Andyp » 16 Mar 2016, 14:34

Certainly a very good test. Thanks for the follow up. Might just have to buy some of that Sikkens Cetol Filter 7.
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Re: External iroko - Osmo UV oil or Sikkens Cetol Filter 7

Postby Rod » 16 Mar 2016, 15:00

I used to use the Sadolin equivalent system before that and reckon Sikkens gives at least another two more years before needing to freshening up.
The only problem is the layers build up and eventually mask any wood look and if you carry out any repairs, the difference is slightly noticeable.
I suppose, like with any paint, you eventually have to take it off and start again but I've never read anything to that effect?

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