Another paint question


I sprayed a number of flat wood items with a water-based paint. I am now spraying on top coats of water-based polyurethane. With the initial top coat, there were spots on one of the pieces, where the topcoat would not wet the surface ... but showed a few very small beads in those spots. After drying, I sanded the surfaces with 400 sandpaper ... making sure the areas that did not wet were well sanded. The second topcoat wet the areas that it had not wet the first time, but now there are a couple of areas on other pieces that did not wet. I am using an airbrush and had to thin the polyurethane before spraying. An ideas as to what is causing this phenomenon?
Chuck
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What kind of wax did you use on your tools?
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I was thinking oil from the air compressor.

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