Right then where do I start,well please for your own sake switch the GAS HEATER OFF when you are spraying...WHY dammed dangerous for starters,if by any chance that you accidentally turn and spray towards the heater then one word springs to mind BOOM or even bye bye Lezz and we cant have that can we.
Right then it odes not sound to me like overspray but it sounds more like DRY spray especially as you have stated that the surface is rough,now how can you prevent this,well there is a couple of ways one has allready been mentioned and is usually the main culprit in causing dry spray..........heat,in other words the paint has dried before it has reached the worksurface.Another cause of dry spray of course is poor technique......Holding the aerosol or spray gun too far away from the item in question,but alas holding it too near will cause runs(another story)The ideal distance to hold a rattle can is approx 9ins away making sure that the next pass of the can is 50% over the first coat thus making sure that the item is wet,any overpray will then be absorbed into the wet area.Another reason is not shaking the can for long enough this can also lead to dry spray.But personally I think that you should also not have the fan on when spraying as this can lead to the paint being blown away from the job.
By all means have the heater on before spraying to get the place warmed up,but make sure hter is not a great cloud of paint in the atmosphere after painting before you switch it back on,any probs then just ask away
Stavros