(Cross-posted from https://chemistry.stackexchange.com/questions/155681/do-positive-results-of-a-lead-paint-testing-kit-imply-that-a-dipped-door-is-now due to a helpful comment.)
We have had some 1930's doors dipped in a caustic solution to remove the paint.
Before dipping, we found that a layer of yellow paint contained lead. It tested positive with: https://www.amazon.com/Testing-Results-Seconds-Suitable-Surfaces/dp/B07NBH7KJJ/
(The testing kit requires the swabs to be dipped in vinegar, where they change color from white to yellow. Rubbing for 30 seconds against safe paint results in no change from the yellow color, while lead paint causes the swab to turn red.)
What appears to be the bare wood of the dipped doors now tests positive when we use the paint-testing kit. (We were testing a small patch of unremoved paint, and initially tested the bare wood by accident.)
Not only does the bare wood test positive, but it turns a much brighter purple than the red color-change produced by the original lead paint.
Is the door now impregnated with lead as a result of dipping?
Or is this just a reaction between the caustic substance in which the door was dipped and the testing chemical?