how well it works is quite dependent on the person doing the application. My friend's work is second to none and he goes out of his way to do the best job possible. I've made several tips for the tube wands he uses; he hates to drill holes so he removes screws and panels to get the product where it needs to be.
I am glad you have someone you can trust. There is no question the person doing the job is all that really matters. Sadly, the majority of rustproofers are not only bad at it, they are also scammers.
I disassembled wrecked vehicles all my life. You see blatant examples every day.
Even vehicles that customers told me were taken back every year, for their "warranty touch ups", are commonly found to have little, or no material in areas you can't see.
Here is a very common example. This fender was removed for replacement. The photo shows nothing had been sprayed on the top rail of the unibody structure that wasn't visible, with the fender on. Rust is clearly evident. And, would have easily been prevented by proper application of the rustproofing material.
The inner skirt, and the strut tower, all visible when you open the hood, was heavily coated. Which is illustrated by all the dirt stuck to it. This is of course, designed to give you the impression, it's all been done like that.
This isn't an just an example of simply missing a spot. This is fraud. And, it is what we see the majority of the time. They usually spray a little on the inside, so it's not so obvious. And then, if they are caught, they can claim they just didn't get enough in there.
Few even try to do it properly. And, very few of them succeed.
Many areas are now packed with insulation, and foam, so it's impossible to get to it them, even if you try.
