As to the OP Question: If he did everything according to Hoyle, then it could be, like Deadbodyman suggested a couple drops of sweat or wrong primer.
For the our situation with the deck lid and dozens of small bumps: I think the only answer is to put it in the Archives under "Unsolved Mystery's" & "Lesson Learned"
. Having said this, I seemed to remember the reason we continued to use one light coat of DP40 thinned down, under the paint. If not for insurance against chemical burn-in,....then because we noticed that the polyurethane seemed
to lay down flatter and smoother. It might not behave the same now that the lead is gone. This was our hobby - turned restoration pro and made our living for twenty-some years.
Glad DP40 no longer has lead in it,...this is one hazard of the trade, that needed to be gone.