Care to walk in the footsteps of giants? Here's a NASA study, albeit 20 years old. http://corrosion.ksc.nasa.gov/91-4835.htm

Short short version: They tested several brands each of several types of paint, over rusted metal.
1. EuAPU: epoxy primer (e.g. Amerlock, Variprime, Awlgrip) under aliphatic polyurethane topcoat (e.g. Awlgrip, Amercoat 450H).
2. Chemical rust converter (e.g. OSPHO, Extend) underneath #1 (epoxy under aliphatic poly).
3. MCU: Moisture Cured Urethane primer (e.g. POR-15) under a 1-part urethane topcoat provided with the system.

They found:
- Chemical rust converters all MADE THINGS WORSE. Paint that worked alone failed badly over rust converter.
- Best performance is from 2-pack epoxy primers under 2-pack aliphatic polyurethanes. Chemistry wins.
- Once the rough rust was off, rotary wire brush didn't have enough impact to be worth doing.
- Needle scalers gave a rough surface that worked best on thick coatings (EuAPU) and badly on thin (MCU).
- Disc sander (16 grit) worked well and was about as effective as a grinder.
- Most of the moisture cured urethanes failed early. One could not move onto the second test because it had congealed in the can. The other did OK but not as good as the EuAPUs.

I would have been interested to see Rustoleum or an MCU under an EuAPU.

That's it! I'm going to stop using rust converter. I have indeed seen some pinholes in an EuAPU over rust converter job I did, and that explains it.

Yeah, I just used Google Shopping to check prices for all those items mentioned. Haven't bought it in a couple of years.


This stuff. http://ppgamercoatus.ppgpmc.com/product ... PDS_AI.pdf
From the sound of things, it may be my dream primer. Right chemistry (epoxy) and designed for overcoating light rust. I don't know what the active ingredients are that make it viable for this.

It can work stand-alone in interior applications. However, you need to topcoat it in exterior applications. Epoxies cannot handle exposure to UV light. They admit that: "Epoxy coatings will chalk and fade with exposure to sunlight." Chalking means failing. For our purposes, those of us without carbarns, there's no such thing as an epoxy topcoat.

In the NASA test, they topcoated Ameron 400 with Amershield, which is a 2-part aliphatic acrylic polyurethane (similar to Amercoat 450.) This combination did extremely well, scoring up with the polyurethanes in the salt spray testing, and had the best score of the bunch in outdoor testing. Notably, it kept its good performance on "uncleaned" surfaces where the surface rust had not been removed.

Oh, and here's something more general on NASA's experience that may apply to you. http://corrosion.ksc.nasa.gov/history1.htm