Long dried rain splotches alters stain, why?


I have experienced this on numerous occasions. My freshly sanded wood gets a drop or two of rain on it. I let it dry. I try staining it and wow! The stain takes up different. Yesterday I had to do an emergency wet sanding of a piece that got a few drops of water on it from my raincoat a week earlier. It was red alder and the spots took up three times as much bartleys stain as the dried areas.
I am mainly curious as to what is physically going on. I know now to cover my wood. I am very lucky that alder does not exihibit the photo-sensitivity that some woods do. I use to have to cover my unstained pieces with black plastic. That was a pain.
Thanks
Tor
ps. I am guessing that the wood pores simply expand, thereby accepting more stain. I wonder if anyone uses this technique to deepen the uptake of stain...
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The "stain" is a bunch of fine particles of pigment suspended (you remembered to stir?) in oil and solvent. If the quality of the surface - rough after setting up with water - is different than the sanded - pressed down and perhaps burnished - more stain will stick to the rough.
Lots of people take advantage of this to break casehardened surfaces and/or set up the fuzz before a final gentle whisk-off with super fine paper.
It's also the reason why you want to sand the glue spot off if you sanded the rest of the wood. Scraping leaves a different surface. Or use dyes instead of pigment stains.
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tor wrote:
Yesterday I had to do an emergency

Emergency? Was it life threatening?
I always cover my wood when in the shop. :)
Dave
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Would dyes be better for production work with red alder? I really enjoy the easy of bartly gel stain, as I have not finishing room. Do you have any experience with that?
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As far as what's physically/chemically going on with rain on wood, other things to consider besides pores opening are ions (salts) dissolved in the rain, and the pH (acidity) of the rain. I don't know off hand how either of these would affect the stain-ability of wood, but it'd be an interesting experiment. Andy
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tor wrote:

spraying nearly dry coats of dye should do the trick. Depending on how strong you mix up the dye determines how many coats you need to apply to get the depth of color required. For red oak, I apply WB dye with a brush because it isn't going to blotch. Dark dyes need some additional help because the oak pores remain light. I'll either apply an oil based stain or a glaze to darken those pesky pores. I always practice on scrap to be sure the net result isn't too dark.
Dave
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On 9 Dec 2005 11:32:22 -0800, with neither quill nor qualm, "tor"

It's proof that staining wood is a crime against Nature. Don't do it, woman!

I'd be willing to bet that the pollution in the air and the dust on the wood combined with the water to make a chemical or mineral coating on the wood which made the stain react differently in those areas. If it was that nasty East Coast acid rain, the PH could have changed enough to do that even without any excess minerals. </SWAG>
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Thanks ya'll
t
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"tor"

You might consider using a hand or scraper plane before actual finishing. In my experience, Adler will raise grain very easily if it get wet. When I use Alder I purposely dampen the Alder to raise the grain then scrape. It makes for a real nice finish.
Dave
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Thats good to know. I will mess around with that. Maybe make some comparison test pieces.
t
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