Hi Alan,
Did these holes appear whilst the paint was still wet, or after it had 'flashed off '? If it was still wet, I would agree with Ian & say its grease or some form of silicon causing the problem. Are you using spray cans or an airbrush/ spray gun to apply the paint? if you are using the latter, it could also be caused by water or oil in the air supply.
i agree with Ian suggestions to rectify this, although I would use a clean, lint free cloth to buff the white spirit off of the hull before it dries, as some times the spirit can just spread the grease around the hull rather than remove it. Also i would prefer to use spirit wipe/panel cleaner/pre-paint (its all the same thing just different names) as used by professional car painters. it usually comes in 5 litre tins, but a trip to a local painters with a tin & a donation to the tea fund usually works, as u don't need much. One word of warning if you use this stuff, is to try it on a small piece of the hull first as it can sometimes wet the paint back up & cause more trouble than u already have. It really depends on the paint you have used, but should be fine with halfords type spray cans.
If the holes appeared after the paint had flashed off, well that could be a whole different story & I would need to know more information before I could give any advice on rectification.
Hope this helps, cheers Glenn