Yeah, follow Wienerdog here.
Paint is wet and has to be, otherwise it wouldn't be soaked up by the wood. After soaking up, the wood is wet, wich causes small plys to errect out of the surface.
Sanding is as important as painting! Use something higher than 180 on
completely dry paint and sand carefully by hand using a sanding block. Doesn't take much effort. After sanding use a old but dry and clean

T-Shirt to remove the dust.
(Didn't want to get off-topic here, but ... couldn't resist to)