What did I do wrong ??

[MIKE28]

I restored a Garden Bench and it came out great
I sandblasted the cast iron sides and back insert
and replaced the wood with White oak. For the
finish I was told to use Watco Teak Oil 
When it was finished it looked great.
And the great look lasted until the first rain,
A very light rain, that only got the seat slates wet.
After the rain I checked it and found that seat slats
where all covered something black that looked like mold
that I could not take off, even using a scouring pad.
I thought that I did everything right, planning the wood,
sanding it and applying the teak oil,  evidently I screwed
up somewhere.

What did I do wrong ??   

[SteveS]

Poor choice in wood.

[fredhargis]

I wouldn't blame the wood, white oak should be good for that purpose. But whoever told you to use the "teak oil" probably could have suggested something better. Not sure what the black is, but I'm guessing it's mildew. My approach would be to remove the finsh (on both the seat and back slats) and start over. My choice of a finish would be untinted exterior paint....but others here have had good luck with some other outdoor finishes, and they will be along to suggest them (and possibly offer a different diagnosis). For the "teak oil', there is no such thing and Flexner wrote a short article in PWW blasting the manufacturers for marketing such stuff.....it's little more than some other product they make in a different can. The Watco stuff is likely just a very thin varnish, or something else. If the untinted paint interest you, here's a little more info on it.

[Cooler]

If you want the wood grain to show through then a good spar varnish.  A good spar varnish is more flexible (and less hard) than regular varnish to accommodate the wood movement due to temperature and humidity changes outdoors.  

But you still want to stick with regular varnishes and urethanes indoors as they are harder and resist wear better.

I've never used these, but they are standard in the marine industry.

https://www.epifanes.com/page/clear-finishes

http://www.pettitpaint.com/product.asp?id=61

[jteneyck]

I restored a Garden Bench and it came out great
I sandblasted the cast iron sides and back insert
and replaced the wood with White oak. For the
finish I was told to use Watco Teak Oil 
When it was finished it looked great.
And the great look lasted until the first rain,
A very light rain, that only got the seat slates wet.
After the rain I checked it and found that seat slats
where all covered something black that looked like mold
that I could not take off, even using a scouring pad.
I thought that I did everything right, planning the wood,
sanding it and applying the teak oil,  evidently I screwed
up somewhere.

What did I do wrong ??   

The black is mold/mildew.  I would power wash it and then treat the wood with a deck stripper/cleaner/brightener to bring the wood back to its original color.  White oak is a very rot resistant wood, but it will turn grey/black if it's not encapsulated in a film finish.  As others have said, the oil finish you used won't do that, you need to use one of the products already mentioned.  I have used Epifanes with good success but I'm sure the untinted exterior paint would work well, too. 

Keep in mind, once you put wood outdoors you are committed to routine maintenance no matter what finish you put on it.  With Epifanes you might get 2 or 3 years before having to address any cracks in the finish and applying another couple of three coats.  Same for any finish.  The only way to avoid routine maintenance is to use no finish at all and let the wood weather to it's natural gray/black. 

John

[Stwood_]

Oak reacts to iron, and turns black. Could that be the problem here? How about some pics, to tell just where the black is.

[daddo]

I have wooden chairs I made subjected to getting damp and sometimes wet under the patio and partial sunlight. I used regular ole poly on them. 3 years on them and they just now need a touch up coat. If it is in the sun a lot, it will wear quicker.
I had worse luck with spar varnish- pealed and cracked the first year.

You have mildew. If you like the old look, just sand and re-coat.

[David G]

I had the same thing happen with a cypress table and BLO finish.  Mold/mildew seems to like the oil finishes.
Careful with the power wash.  It is easy to gouge out the soft wood parts.