The straightforward answer is, the only way to regain lost clear coat through polishing is to re-spray it with more clear coat.
There are a lot more factors with what you're asking, though. The several questions you should be asking when polishing a car are:
- Does my paint even have a clear coat? (Is the paint used a single, dual, or three stage paint?)
- How thick is the paint and/or clear coat on my car?
- How much clear coat is removed during the polishing process?
- Are there any other factors I should consider when polishing?
Every manufacturer can use a different painting scheme, which means, any given car manufacturer will have a different amount of paint on a car after the production process has concluded. In order to know how much paint you have (or have left), you need to use a Paint Thickness Gauge (PTG). While they are not perfect, they can give you an idea of how much is there.
According to this forum post, the following is a guide to what is considered paint thicknesses and what is good:
- 200µ + can be expected on older cars that have been hand painted or a re-painted vehicle
- 100 <> 200µ - average paint thickness
- 80 <> 100 µ - thin paint
- < 80 µ - very thin paint
(NOTE: µ = micron; 25.4 µ = 1 mil; 1 mil = .001")
It also says on the webpage:
Using a medium abrasive polish and a rotary polisher will remove approximately 2.5 - 3µ (0. 1 Mil) from the paint surface, which is typically four passes at 1500-1800 RPM; however many variables such as polish/compound and speed / pressure used that may affect the paint removed
If you have reservations about the amount of paint surface removed or the amount of paint remaining the use of a paint thickness gauge (PTG) is arbitrary. There comes a point when you must judge wither removing a scratch will compromise the clear coat and / or UV protection, if so you'll have to live with the imperfection
The poster also states:
A paint thickness reading of > 100 µ (Microns) is reasonably safe for polishing. 80-90 µ, I wouldn't use anything stronger than <2000 grit polish, 70-80 µ <2500 grit polish and under 70 µ use a glaze. The readings tend to vary from panel to panel and are thinner towards the panel edges and any seams.
If you are that concerned about the thickness, yet would like to see the shiny surface, you may want to use the following guidance:
The newer coatings available like synthetic polymers are a cross-linking thermoplastic, its cross-linking process attaches the polymer with covalent bond that becomes part of the surface of the material it is attached to, which in effect becomes a secondary protection for the clear coat, in fact a relatively inexpensive (when compared to repainting) renewable sacrificial coating.
Silica (AQuartz) or reactive resin hydrophobic coatings (OPT Opti-Coat); think paint sealant that has greater durability and scratch resistance (9H) something that also provides a self-cleaning protection, with a durability on a timescale measured in years rather than months, these coatings add a measurable protection of 2-3 µ microns to the clear coat.
Things like this will help your paint live longer without the need to polish off the clear coat.