Today's clearcoat polyurethane clears are porus, they are like a sponge, just look at the "highly magnified modern clearcoat system" photo on
http://www.autoint.com.
This is not a "manufacturered" photo, it is the same used by the vehicle manufacturers to train their employees who deal with paint concerns.
Not "knocking" clay here, it has it's place in vehicle care.
If you ever had a basic chemisty course,then you understand how acids work. The "bumps" are usually due to the acids that have penatrated into the "sponge" of the paint that you will see. When water is introduced (rain, washing, dew,etch) activates the acids, they start eating at the resin system of the paint system and the "bumps" start.
No clay can get those acids out, may make the paint feel "smoother", you feel better, but just like getting battery acid on your Levis, it will eat the jeans up , it will eat your paint up, just a matter of time.
Clay away, the bumps will come back. The "swelling" will return, just a matter of time.
Have a session this week with a car dealer, going there at PPG request, the dealer is having a serious issue with his new car inventory, seems the "kids" have been "claying" the vehicles which exhibit "bumps" that they new car buyers have been complaining about, but they keep coming back after a short time period.
The dealer has been repainting almost 25% of sold vehicles, and that is the issue I will be addressing.
Really quite simple once one accepts basic chemisty.
Stop treating the symthoms, treat the cause!
