hmmmm... yer very rite bro.. this happens... but, my car has not been at a service station for over two months... :S:S:(
and every time I wax it, the first thing I do is to check "THE FEEL" on the surface... (H)
If it is by any way oily,I scrub it off and then apply the wax.
BTW at the time of rain, the car had like 3x the dust in the following picture and after a continuous shower of 3 hours, the dust was almost washed off...
if it were oil, it could've caused a havoc... 


and yar, :(, today the car started giving "shhhhhhh" sound from the engine continuously... gonna take it to take mechanic... if may be i really need to replace the timing chain by now... :'(