[911-Hotrod]

Etch primer was designed to be a much faster method of applying paint to bare metal for the automotive body repair industry. It is not as good as epoxy or moisture cured polyurethane, but time is money in the body repair business, and they aren't doing a "forever" collector car either.

I have no knowledge of what POR-15 company may have come up with to apply to their POR-15 moisture cured polyurethane that would require no sanding or surface prep first. If you understand the high chemical resistance of the polyurethane and that most solvents used to thin paints will not effectively attack the polyurethane to allow a top coat paint to properly adhere to the polyurethane, then you would believe as I do that the only proper way to adhere a top coat to the polyurethane would be to give it a textured surface for the top coat paint to adhere to the micro scratches and obtain a mechanical lock to the polyurethane base coat. The same is true for epoxy.

I personally would never apply any paint over the top of either epoxy or moisture cured polyurethane without providing a textured surface first. There maybe some epoxy formulation that POR-15 company has come up with to stick to the very smooth surface of the POR-15 polyurethane, but it will never stick as well as it would if the surface has micro scratches. Unless there is no other way, I would skip applying their epoxy over top of their polyurethane. Just texture the surface and go straight to sanding primer or top coat color.

Chassis Saver has two benefits over POR-15, the aluminum flakes which provide a better barrier to water molecule migration thru the material and the cost is much lower.

Seam sealers are mostly polyurethane formulations. There are two-part types and single part types that cure by moisture or by solvent evaporation. There are also rubberized epoxy formulations. Most seam sealers are flexible once cured. This would make them more difficult to texturize prior to painting over them, but most of them can be directly painted over and still have adequate adhesion.

I have painted with Chassis Saver over the top of rubber tires, rubber moldings and plastic parts such as bumpers, leather and fabric. I have never had it not stick to anything and everything it was painted on, even the overspray.

I repainted an F550 flatbed truck where the steel flatbed had a very thick layer of rust which required an air powered needle chisel scaler to chip it off. The rust came off in thick scales. I then poured on muriatic acid to dissolve as much of the remaining surface rust as possible. I always remove as much of the rust as possible even though I know that thin rust could be painted over by Chassis Saver, but why do an inferior job? I then used phosphoric acid to prep the steel prior to applying Chassis Saver. I then applied an industrial enamel tractor paint before the Chassis Saver reached full cure. It worked out great except some of the over spray from the Chassis Saver floated as much as 8 feet away and landed on the front of the truck's cab where I had not masked off. I had only masked off 4 feet from the area that I was spraying. Even though the cab paint had not been cleaned in years and had lots of dust on the surface, the Chassis Saver stuck so well that it would not come off without being sanded off. I had to color sand and buff the cab paint anyway, so it worked out. Just be aware that moisture cured polyurethane overspray does not flash dry in the air like most paints that settles as dust on other surfaces and wipes away easily. The floating overspray will stick to anything it lands on and will be permanent if it is not removed with solvent before it cures.

I don't see any reason why the moisture cured polyurethane would not adhere well to the seam sealer. I would still recommend that you use a scotchbite pad to take the glossy sheen from off of the seam sealer surface prior to painting over it if you want the best adhesion possible. Moisture cured polyurethanes do have great stick to almost all types of materials, just like glue sticks to most surfaces, but the more surface texture there is, the better the stick. Don't "prime" over the seam sealer prior to applying the moisture cured polyurethane. There is no primer that would stick any better than the polyurethane.

I would recommend using either epoxy or moisture cured polyurethane on any and all metal surface instead of etch primers. They are far superior to any etch primer. If you want the best adhesion, then produce a micro textured surface, then use a phosphoric acid metal prep prior to applying the epoxy or moisture cured polyurethane. Chassis Saver is less expensive than epoxy primer. The phosphoric acid prep produces a microscopic crystalline structure that looks like a forest of trees when viewed with an electron microscope, that provides micro anchors for the paint to attach to. It also gives the surface a pH that is acceptable for the paint chemical compatibility.

All of the paint manufacturers have tech support people that you can call directly because most paint store people are not that good at the chemistry and properties of the paints, some don't even have any actual hands-on experience. Just remember that most people in the paint and body repair business gain their training by having their boss teach them. Who taught the boss? His boss of coarse. This hand-me-down training process passes on many bad ideas and bad procedures. Avoid that by going straight to the technical experts at the paint manufacturers directly. I come from the science and engineering side of things and it is my nature to research, experiment, test, test, and then test some more because I am a perfectionist and want the best possible for my stuff.

Not all paints are created equal. Automotive paints are getting really expensive. I have paid from $50/gallon to almost $400/gallon. It all depends upon your requirements and budget. The best paint I have used is made for the marine luxury yacht market where the paint needs to survive 20+ years of outdoor exposure such as Alexseal. There are lots of choices. I have read that there are some new developments regarding moisture cured polyurethanes that are very UV resistant and can be used on sun exposed surfaces, even as a clear coat. These developments are being prompted because the paint manufactures are trying to get away from solvents to reduce VOC emissions. Do your research.

You are doing a great job. I tell people that anything is possible if you have the right knowledge, tools, materials and desire. You are proof of that statement.