Oak garden bench finish

[Buckeye]

I am finishing up on making a traditional oak garden bench for my folks (too many mortice and tenons but will post some pics when finished!).

I have a question about the best finish for this. I don't mind the colour fading. I don't want a really plastic looking varnish.

I would appreciate any thoughts or advice on good outdoor finishes that would at least offer decent protection from moisture/mildew type effects. I know Osmo do a exterior oak finish, has anyone used it?

 

[MikeG.]

None of them are great. Oil is pretty much your only choice. Don't whatever you do even think about using a varnish.

Osmo has a UV oil which works OK for oak, but isn't recommended for horizontal surfaces. Fiddes also has an exterior UV oil. I don't know anything about it. Both of these will reduce the rate at which the oak goes grey. Others suggest multiple coats of (initially thinned) tung oil, but this won't prevent greying. If you use an oil, remember the old adage....once a day for a week, once a week for a month, once a month for a year.

ETA Osmo recommend Natural Oil Woodstain

 

[Buckeye]

None of them are great. Oil is pretty much your only choice. Don't whatever you do even think about using a varnish.

Osmo has a UV oil which works OK for oak, but isn't recommended for horizontal surfaces. Fiddes also has an exterior UV oil. I don't know anything about it. Both of these will reduce the rate at which the oak goes grey. Others suggest multiple coats of (initially thinned) tung oil, but this won't prevent greying. If you use an oil, remember the old adage....once a day for a week, once a week for a month, once a month for a year.

Thanks Mike. Will stay clear of varnish.

Hadn't heard that adage about frequency of applying oil before. Certainly would have done a few coats but didn't know that many would help.

 

[MikeG.]

Careful to distinguish between oils (such as Tung oil) which do require lots of coats, and wax-oils, such as the Osmo and Fiddes products (did you see my edit?) which don't require so many coats. Follow the manufacturer's instructions on the latter.

 

[custard]

You said you don't mind the colour fading, so why use any finish at all?

There have been endless posts on outdoor finishing, but one guy once said something really original. His argument was that the microscopic bugs that cause that nasty green, slimy effect on timber are actually feeding on the nutrients in oils and varnishes. So if you use oil or varnish you're committing to a lifetime of regular re-finishing every couple of years. Because if you stop you'll then have several years where the item looks worse than if you'd never applied anything in the first place. Eventually the bugs will have consumed all the traces of the oil and then the timber can finally turn silver.

I'm not qualified to say if that's true or not, but it certainly fits with my experience. With a durable timber like Oak I'd say don't apply anything and just learn to love that silvery grey look!

 

[Phil Pascoe]

Whether you use a finish or not I would get some sheet lead or copper and clad any horizontal or near horizontal end grain - most of the decay I've seen in outside joinery over the years has been cause by water tracking down through the end grain. I regularly pass two oak gateposts I put in in 1975 that are still perfectly sound - I clad the tops with ally sheet the day I put them in. A tidy bit of lead or copper (beaten?) can look good as well.

 

[MikeG.]

Copper will stain the oak green. Tannins in the oak will eat away at the lead. Aluminium is a better bet.

 

[Phil Pascoe]

Yes, I didn't think of the copper staining. I've used lead successfully ............. but only on softwoods. I didn't know tannin would affect it.

 

[Buckeye]

You said you don't mind the colour fading, so why use any finish at all?

There have been endless posts on outdoor finishing, but one guy once said something really original. His argument was that the microscopic bugs that cause that nasty green, slimy effect on timber are actually feeding on the nutrients in oils and varnishes. So if you use oil or varnish you're committing to a lifetime of regular re-finishing every couple of years. Because if you stop you'll then have several years where the item looks worse than if you'd never applied anything in the first place. Eventually the bugs will have consumed all the traces of the oil and then the timber can finally turn silver.

I'm not qualified to say if that's true or not, but it certainly fits with my experience. With a durable timber like Oak I'd say don't apply anything and just learn to love that silvery grey look!

Thanks Custard; worth considering. I had thought a finish would help with keeping the slime away.

 

[Buckeye]

Whether you use a finish or not I would get some sheet lead or copper and clad any horizontal or near horizontal end grain - most of the decay I've seen in outside joinery over the years has been cause by water tracking down through the end grain. I regularly pass two oak gateposts I put in in 1975 that are still perfectly sound - I clad the tops with ally sheet the day I put them in. A tidy bit of lead or copper (beaten?) can look good as well.

Thanks Phil, wondering if it would be worth epoxying the end grain bottom and last half inch of the 4 legs to prevent water ingress as the bench sits on the ground?

 

[MikeG.]

Much better to raise the feet up off the ground if you can. Fifteen or 20mm is ideal, but even 6 to 10mm works. This allows water to drain off, and the bottom of the timber to dry. The design of this would depend on whether the bench was permanently fixed in place or not, and on the surface on which is sits.

 

[Phil Pascoe]

I did the bottoms of the legs of my softwood garden table with Flashband - it stood on the lawn and was still solid sixteen years later when I sold the house.