'Blooming' is an effect caused by moisture being trapped in the nitrocellulose lacquer finish as it dries. This most often occurs when the lacquer is sprayed in cold or humid conditions, or if there is a temperature difference between the lacquer and workpiece. The result is a cloudy white appearance to the lacquer.
It looks dramatic and disastrous but, thankfully, is easily fixed. The cloudy appearance will vanish, and the lacquer return to clear, if the moisture is released.
Nitrocellulose lacquer burns in to the coats beneath it. The solvents in the lacquer when sprayed redissolve the very top of the coat being sprayed on to, and the lacquer then dries to a single homogeneous layer. This is in contrast to modern curing lacquers, where each coat sits on top of the last, building up like the layers of an onion.
To cure blooming key the bloomed lacquer to 600 grit, then apply a light coat of lacquer over the top. This will re-dissolve the existing lacquer allowing the moisture to escape. It should now dry to a clear finish. It is important that this process is performed in a dry, room-temperature environment.