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Good Afternoon,
I am a manager at a large horse facility, and every year we hand sand and re-polyurethane our stalls. We don't sand to the wood, just enough to remove the gloss to help the new coat stick. We don't want to sand deeper than that, because re-staining all 8 barns is too big a task.

We have run into some problems, however. Due to so many old coats, as well and changing weather conditions, there are areas that aren't well adhered any more. Putting additional polyurethane over these results in large chunks peeling off a few months later. My thought is to use a scraper, but I'm employing unskilled students to do this, and I'd like to limit their ability to gouge the walls too badly (hence hand sanding versus mechanical). Are there any thoughts on how I can get a good bond to the walls with the new polyurethane using failproof tools?
 

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Good Afternoon,
I am a manager at a large horse facility, and every year we hand sand and re-polyurethane our stalls. We don't sand to the wood, just enough to remove the gloss to help the new coat stick. We don't want to sand deeper than that, because re-staining all 8 barns is too big a task.

We have run into some problems, however. Due to so many old coats, as well and changing weather conditions, there are areas that aren't well adhered any more. Putting additional polyurethane over these results in large chunks peeling off a few months later. My thought is to use a scraper, but I'm employing unskilled students to do this, and I'd like to limit their ability to gouge the walls too badly (hence hand sanding versus mechanical). Are there any thoughts on how I can get a good bond to the walls with the new polyurethane using failproof tools?
Probably what has happened is some previous coat the poly wasn't sanded well enough. There is always problems with adhesion using polyurethane anyway. I believe I would have them use a putty knife and skin off all the loose poly you can and then sand what is left with 220 grit paper and recoat it as you have been doing. The only real foolproof solution would be to chemically strip the finish down to bare wood and start over.
 

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You might want to get a few palm sanders and some low grit paper and hit the spots that you are having issues with. This would be faster than sanding by hand, but you would have to tell them to watch what they are doing because the could easily push to hard and get to bare wood.
 

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The adhesion problem my be related more to the horses and less to the poly. In a stall you will have a lot of fine dust that will deposit into any pore it can find. The "dust" is packed full of chemicals like ammonia and oils from the horses coat that can affect your finish coat.

I am sure you are washing it down before you start sanding, but you might consider washing it again after sanding with something that might neutralize the chemicals in the dust. Short of taking the surface down to bare wood and re staining I not sure how to ensure you have removed all of that "dust". A fairly low power pressure wash might help or scrubbing with a stiff brush and then rinsing.
 
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