I'm a rookie, i've used the PPG and Omni epoxy. They suck compared to the SPI. Try sanding the PPG stuff after a MONTH, it will gum up just like you sprayed it 2hrs ago. No matter what you do, PPG will not lay smooth, even if you reduce it twice what it calls for, still peely. SPI, lays smooth, sands super easy.
As far as the SPI site that Deadbodyman refferred to, it is tough to post over there. I made one comment about how a "dealer painter" did a hack job on a friends car, they jumped all over me, PM'd me about it. All i was saying that if you don't take pride in your work, don't do it just to make a buck. Makes all the others in the business "seem" like hacks to others outside the business. It stereotypes "all" painters which, isn't true. Now if i have an SPI question, i just call Barry directly. He is always helpful and guides you thru the process.