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I made a coffee table out of pallet wood. If I had to guess there is at least 3 different kinds of wood here and I don't know for sure which they are.

The white you see is home made chalk paint made from drywall mud and white acrylic paint. It had about 2 coats of paint. To protect and finish I choose polycrylic.

Somehow one of the blocks stained the paint yellow. I saw one other block like this but it was not as defined. It reminds me of painting the walls of a former smokers house where the tar seeps through the paint.

Foot of my coffee table

I am still new at finishing and I should have realized that I was applying too much polycrylic as you can see the dripping. While that was bad it did show me something. If you see on the right hand side of the block the yellow colour does not go all the way up the block. There is a drip there that goes from white to yellow. That tells me the finish was not the source of the issue? Perhaps just exacerbated it? Most of the wood pictured has a coat of polycrylic on it and that is not a test coat.

I started looking more for the colour and I found one more block that would have been from the same pallet that had a little yellowing.

Everyone of those blocks you see had pith so it should be a fair amount of heartwood. Also the spots where you see a little bare wood was done on purpose prior to polycrylicing (If that is a verb).

Why did the finish turn yellow in those few places? I think it has something to do with the wood itself.

As an aside I just sanded down the finish and most of the paint in that area. The table was meant to look distressed anyway. Still curious what happened so I can try to prevent it or at least be aware of it.

Matt
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  • One note about "pallet wood" in general... You don't know if the pallet was used to hold bananas or if it was used for industrial pesticide. It could've had anything spilled on it, so treat it with caution and seal appropriately. Not saying that's what your yellow stain is... just a general word of caution. – JPhi1618 Oct 29 '15 at 13:03
  • @JPhi1618 Good advice yes. I don't believe that is my issue since I closely follow this guide: http://woodworking.stackexchange.com/questions/342/what-do-i-need-to-know-when-looking-at-pallets-for-wood – Matt Oct 29 '15 at 13:47
  • and don't forget it could be some fungus living in the wood or producing a chemical that acts like the tannin in the answers below. If you don't know the history of the wood and the finish is important, you need to try and seal it. – bowlturner Oct 29 '15 at 16:01
  • The American in me is really wanting to edit the title to say "color" instead of "colour." :-) – grfrazee Oct 29 '15 at 16:07
  • @grfrazee I was wondering if someone was going to try. – Matt Oct 29 '15 at 16:57

2 Answers2

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Why did the finish turn yellow in those few places? I think it has something to do with the wood itself.

I actually just had a similar conversation with my dad on this topic. He's a polymer chemist and works for a paint company.

What you might be experiencing is tannin staining from the wood underneath the paint. All wood has some level of tannin. With your pallet wood, you have a real crap-shoot with what wood you actually have, so its tannin content could be quite high.

What you need is a paint with a tannin blocker in it. Kilz Premium1 is currently the best primer on the market for this purpose, and it has a special shellac-based polymer in it that inhibits tannin staining. Shellac is the best tannin-blocking material known to man2 (thus far).

If you're set on using your homemade chalk paint, you might consider applying a seal coat of shellac beneath the paint to help mitigate the tannin staining. Then lightly scuff the shellac coat with fine sandpaper or steel wool so the paint adheres better.

1 Full disclosure, my dad does not work for Kilz.
2 See page 5 of this study.

grfrazee
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3

All woods have what are called in the trade extractives and these tend to be soluble compounds which can be liberated from the wood if it gets wet. You can see this as a "coffee stain effect" when you accidentally drip water or another solvent onto the surface of bare wood and it is allowed to dry on its own.

It's usually not a factor when finishing because the wood gets wet fairly uniformly and not sopping wet as when a pool of liquid sits on the wood's surface for some time.

Another possible cause in this case (or additional cause) is that pallet wood is very much an unknown quantity, we only guess what it's been through when we snag a pallet and break it down for the wood; so this could be something in the wood that wasn't there originally, or a consequence of the weathering the surface endured while it was in use.

Graphus
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