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Old 05-13-2011
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Thinned Varnish turned Teak Black

I'm replacing some and refinishing other teak on my C&C Invader. Some of the wood is 40 years old but is still firm. It was painted with Cetol which I've sanded off. I used Ajax to clean the wood per a master boatwright's post I found and rinsed it very well.

I thinned my varnish about 20-30% for the first coat with mineral spirits and painted it on. It turned my teak black. I thought it was just the wet mineral spirits and it would dry out and go light but it did not. I guess I have to now try to sand that thinned varnish (which soaked in pretty deep I guess) off again.

Ive attached a couple of photos. You can see in one of them that there is a newer piece of teak (on the right) up against the older teak. The newer teak performed perfectly and stayed light. The older section (on the left) went almost black.

Any ideas why this happened?

Edit-1:OK, I can't post links or photos until I have 2 or more posts. Look for a new Post with a similar subject.

Edit-2: That didn't work so lets try this. Here are the links. You'll need to fix the front end of the URL. I'm sure that severely limits the number of users who will take the time to help me but I had no choice. Sorry for the inconvenience. I've never see that rule on a Forum before. I guess it thwarts SPAM posts.


h t t p : / / nvision4d.com/public/IMG_1080_s.jpg
h t t p : / / nvision4d.com/public/IMG_1075_s.jpg

Last edited by RobH2; 05-13-2011 at 09:40 AM.
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Old 05-13-2011
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what varnish are you using? It might not work well with mineral spirits and needs a special thinner designed for the varnish.
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Old 05-13-2011
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I'm thinking it was the thinner used too. Products have their own thinners for a reason...
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Here are Pettit's instructions for varnishing bare teak :

1. Sand the wood smooth with 120 grit production paper to open up the grain. Wipe the surface thoroughly with Pettit 120 Brushing Thinner in an effort to
aggressively remove as much oil as possible.
2. Apply a generous coat of Pettit 2018 Clear Sealer. After an overnight dry, lightly sand the surface with 220 grit sandpaper and wipe it clean with a rag
dampened with Pettit 120 Brushing Thinner.
3. Apply at least four coats of Z-Spar 1015 Captain's Varnish. Let each coat dry overnight, sand with 220 grit sandpaper, and clean off sanding residue with a
tack rag before applying the next coat. Sand the next to last coat with 400 grit production paper prior to applying the final coat.
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Old 05-14-2011
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That's weird. I've never seen that happen, except when 'cleaning' bare teak using lye-based products such as Snappy Nu Teak.

All i can offer is you're going to have to undo and redo to get it right. And a rubrail butted against what i'm assuming is an aluminum toe-rail is especially tricky, because it constrains your ability to use products like Snappy Nu-Teak to bleach out the wood once you remove the offending finish.

Assuming that's the plan - undo/redo...I do suggest you 'clean' the teak with a product like the one above after getter back to bare wood....but be careful as these products will eat aluminum and paint (like if it were to run down your topsides). But that is the best way IMO to prep bare teak before applying finish as these products will remove the natural oils from the surface of the wood and provide a much more adhesive bond between the varnish and the wood.

Good luck.
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Old 05-14-2011
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Sometimes older teak will become blackish when freshly varnished. This sometimes happens after aggressive power-sanding then applying a urethane based varnish.

Most of the time this blackish dis-coloring will quickly fade to 'natural' color with time if left in full sunlight (and covered with saran wrap if you haven't quite built up sufficient coats of varnish).

If your in a hurry, suggest you sand back the 'old' piece to bare teak, bleach with oxalic acid (simple 'wood bleach' from a paint/hardware store) then apply an acrylic 'sealer' before varnishing. If you originally used urethane varnish apply the first 1-2 coats with oil based; or vice versa.
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Old 05-15-2011
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With old wethered teak the softer parts of the grain will often be indented or deeper than the hard parts of the grain. After sanding although the teak will be a light brown or tan deeper parts of the grain can still be grey and will darken when varnished. A good wood bleach will help but not totally eliminate the grey. You need to find that happy medium between sanding too deep where too much wood is removed vs not enough.

Unless you have either unlimited money or time an older boat will be hard to get perfect. But like old homes signs of use with good maintence increase character and will create that well cared for look without perfection.
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Old 07-25-2012
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Re: Thinned Varnish turned Teak Black

Hi RobH2, I to own an Invader 36. These are beautiful classic yachts. It would be a good idea to make an inventory of boats and owners. I know of another Invader on lake Champlain. Add that to the three you already know about:-)
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Thinned Varnish turned Teak Black

I agree with Rich H and had the same blackening happen after sanding and using polyurethane varnish on the badly worn rails of my C&C 24. I hope to strip it and recoat with Sieken's Cetol. Worked on my Searay teak which had too many coats of cheap varnish and pealed. Good luck
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Old 07-25-2012
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Re: Thinned Varnish turned Teak Black

The olls in the old teak "carbonize" only way to get it out is with bleach and even power washing. Well, that's how I brought back some pieces that were old. Of course you can destroy the wood if you don't use a fan spray setting on the nozzle.
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