It really comes down to 5 things:
1. Blast media
2. Blast PSI
3. Nozzle size
4. Blaster experience
As an example, we blast using 40-70 (fine/med) crushed glass which is softer than many media that are commonly used, like coal slag (aka "black beauty"). We also blast between 40-70 PSI max, which is very low. We use small 1/4" nozzle (small for a commercial-sized pot). The angle (user experience) that you are using is also a big factor. The down side is, #1-3 above translate into taking more time to blast. Blasting a full car body correctly takes quite a bit of time and patience.
From an example perspective, we commonly talk to people that have been told by sandblasters (that don't typically focus on car bodies) that they can do a whole body in anywhere from 3 (the lowest I have heard) to 5 hours. This may be true, but to accomplish this you have to use a very aggressive media and/or a high blast PSI...neither which is good for body panels. The aggressive "profile" that is left on the metal, warping, heat, blow-thru are all common issues using the "strip it fast" strategy. We have actually had several car bodies in the shop that have been incorrectly blasted and require panel replacement (typically warped roofs, hoods, and qtrs).
By comparison, it takes us about 12 hours to completely strip a shell (inside/outside/underneath).
When choosing a blaster...ask ALL of the above. Also make sure that they have done several cars and you can call references. Nearly all sandblasters will say, "we can do that." Be leery of pricing under $500 for a full body and time estimates below 8 hours.
I hope this is helpful?