Firstly - some context - Clear coat contains an ultraviolet (UV) inhibitor and it gives your paint its shine and depth. The reason behind applying clear coat over your base colour is to prevent your base paint from oxidizing (turning grey/white).
Since clear coat is a protective layer that also makes the car look better it would be logical to apply it at the very end of the process – this would guarantee that all layers have reached their ideal thickness and you would not risk damaging them while buffing. So buff before.
Will clear coat hide imperfections?
No, but it will hide scratches and provide a protective layer so the car is not constantly in contact with dirt and dust. It also provides a glide effect when you put down your applicator or microfiber for waxing.
Clear coat does not fill in any imperfections since they are all still visible just under the clear coat. However applying more coats of clear can help fill in small imperfections that are not yet visible through the base colour.
Applying too many coats of clear, however, can cause an uneven surface that shows off all of your mistakes! One popular method to get rid of these big accumulations of clear is by wet sanding them away.
What causes clear coat to bubble?
There are multiple reasons that can cause your clear coat to bubble, but the most common one is not applying enough base colour. Clear coat should never be applied on top of just wax or any other product for that matter.
Even if you do not plan to apply another layer of paint over it you should properly prepare the surface by sanding and/or polishing before applying clearcoat in order to get rid of any noticeable imperfections that will be visible through the clear.
Any old wax, dirt or polish residue left behind could also lead to trouble when trying to apply clearcoats. Other causes include natural oils coming up from the paint which makes the surface “tacky”, leaving a film between layers instead of letting them bond, lack of flash time between coats, too wet or too thick products during application, products not compatible with each other, using spray cans for base colour or clearcoats and not properly shaking the can before use.
Will primer improve the quality of my clear coat?
Primer is used to prepare a surface for painting, but it can also be used as a base colour. Applying base colour over old faded paint is not recommended.
If your car's paint job has faded or oxidized you should first apply a compound or polish to bring back the shine before applying any new layers of paint, so that all colours are consistent with each other.