I have a small patch of rust on my rear passages door. Would the 3 step process work on that? Another question is could I apply the rust seal product to some of the existing paint and than just paint the over it?

Yes. The System Sampler (http://www.kbs-coatings.com/system-sampler.html) is great for patches of rust on cars. You will want to coat over the RustSeal with your touch-up paint. And yes, RustSeal will stick to other paint as long as it is cleaned and scuffed.

You can find complete directions for RustSeal application here: http://www.kbs-coatings.com/rustseal-directions.html

We also have very helpful instructional videos for the application of RustSeal that you can watch online for free here: http://www.kbs-coatings.com/instructional-videos.html

In addition, we also have a Flow Chart for the application of RustSeal here: http://www.kbs-coatings.com/rustseal-flow-chart.html

We also have our How-To Section which has two series of common step-by-step procedures for the application of RustSeal found here: http://www.kbs-coatings.com/how-to-use-kbs-coatings.html

And before you begin your project, it is always a great idea to familiarize yourself with our RustSeal Fix-It Guides found here: http://www.kbs-coatings.com/fix-it-guide-rustseal.html