im sorry but i cant guarantee that the paint won't crack, it depends on what is under your paint, if there is any bog, well i can tell u now there is an 80% chance it will, if there is bog u can tell on the spot after checking your guards. if there is no bog, then your pretty safe a good 99% chance nothing will happen. 1% as there is a risk in everything, but u can tell if the paint is going to crack before u go to far.
the drama comes in when u start pumping the guards. it can be done and ive done a few cars like that, can pump guards up tp 25mm, but costs more due to time. there is a risk with it though. i will be rolling and pumping my guards about 25mm and will take before and after shots during each process.
it all comes down to the way you approach the job, be professional take every precaution and advise the owner if there is a risk of the paint cracking then if the owner is happy to take the risk of cracking then nothing can be said if the paint cracks.
saying this, i have done about 15 cars, and only 1 was a problem because it had bog. i told the owner and he told me to continue. it cracked slightly but was right under the guard.
all i can do is be honest with you guys.