If you spray anything (SPI included) that does not adhere to the metal properly due to contamination then moisture can allow for rust. Without proper prep you get failures.
Now, in the case of a cowel you need to decide if its worth cutting up a cowl that looks good to protect it, knowing that you may end up with a different set of issues.

It honestly amazes me how people can be so ignorant in this day and age? I mean seriously, in general Epoxy is good enough to use on a shop floor, or to make a pool liner, but for some reason people refuse to blame poor installation for poor results when using it on a car? A big WTF here?

 

We "tin canned" the ends of the cowl and checked for rust or damage. We were lucky there was none. Painted the inside with Rust Encapsulator, applied seam sealer and then top coated SPI epoxy by brushing it on. If I had it to do over, I would just use SPI epoxy and seam sealer on top of it.
Pictures and write up : http://chris66dad.tripod.com/id18.html

 

Thanks to everyone...now it's time to get that cowl opened. Chris66dad, I am thinking of going with your route vs dealing with the spot welds. So...rust encapsulator and seam sealer...game on!!!

 

So, you want to use floor paint on your Mustang? "Epoxy floor paint" has absolutely nothing in common with "Epoxy Automotive paint", that is absurd.

I tried really hard to not respond to this post again. In the past, I promised that I would not try to inject logic where it was not wanted. But I just have to….

Poor surface preparation does cause paint systems to fail, that is obvious. Often, what I have seen was a failure in the system used to apply the paint products. Contaminated equipment happens but more often I have seen improper equipment used. High humidity environments without a proper desiccant/moisture control system are common. In the process it is common to see failures due to improper sealing of foundation materials to prevent chemical reactions from occurring.

Paint materials carry an ASTM rating. It is what class of tests the product passes. The products are defined by what tests they pass. The tests are usually a set of performance parameters that define what the product is intended to be used for. A “primer” is a “primer”, not a base coat or a color coat or a finish coat. Some primers can be sealers, but only if they pass the proper tests. It is a physical impossibility for a “primer” to pass the test for a “finish coat”. A primer does just that, it primes (prepares) foundation materials so that the base coat has a consistent surface to adhere to. The tests for Automotive type paints are fairly strict on these matters.

It comes down to simple ASTM ratings, not hard to look up. The rating # used to be on all paint cans, but now you sometimes have to request the testing reports from the manufacturer. Look it up, lots of ASTM ratings are available online.
Once you do a little research, the reasoning is obvious.