Every restorer eventually meets a piece of steel that cannot be blasted. The floor pan of a car that still has its interior in, a cast-iron grate that would lose its detail under grit, a frame rail you can reach with a brush and nothing else. The usual answer is to grind off what you can, prime over the rest and hope. Six months later the rust comes creeping back out from under the bolt heads, and the paint lifts with it.
Three Cortec case histories from Minnesota, spanning 1997 to 2022, show a different answer: convert the rust that is left into a stable, paintable layer with CorrVerter, then topcoat with a water-based acrylic. The products and the rules are the same ones we sell to fleet shops and steel fabricators. The difference is that a restorer can do the whole job in a heated garage in January without a respirator.
A 1947 sedan, checked again 14 years later
Cortec Case History #769 (2008) belongs to a collector in Minnesota with a 1947 sedan that had rusted underneath after years of winter driving. He wanted the front underbody cleaned up and the rust stopped, and he wanted to passivate the rusty floor inside the cabin without pulling the car apart.
The sequence was straightforward. He scraped decades of grease and road grime off the front wheel drums, shock absorbers and undercarriage, then cleaned and degreased the bare surfaces. He spray-applied CorrVerter Rust Converter Primer, using just enough to cover the rust and a brush to get into the corners and seams. After the CorrVerter had dried and cured, he sprayed VpCI-386 Black over it as the topcoat. The interior floor got the same two products.
The work was done in a closed garage in the middle of a Minnesota winter with the heater running, and the point he made afterward was that neither product was flammable or gave off solvent fumes. Both are water-based. In December 2021, almost 14 years later, he was working on the brakes and found no rust creep coming out from under any of the nuts and bolts, which is where a primed-over repair usually fails first. The underbody was still clean, solid and rust-free.
A Civil War cannon for outdoor display
Cortec Case History #193 (1997) is the opposite case: a job where blasting was possible and the challenge was the exposure afterward. A Minnesota antique-restoration shop had a Civil War cannon more than a century old that was going on permanent outdoor display, and it needed a coating that would stand up to Minnesota weather year-round.
The shop sandblasted the cannon to remove rust and old paint, sprayed VpCI-373 as a primer, let it cure for 24 hours, then sprayed VpCI-386 Black and VpCI-386 Blue as the permanent topcoats. It chose the system because primer and topcoat carry both contact and vapor-phase corrosion inhibitors and because all three products are water-based, removing the fire and explosion risk of the solvent-borne inhibitor coatings it had used before. The case rates the VpCI-373 and VpCI-386 system at five to ten years of outdoor protection.
Two rusty grates and a radiator cover
Cortec Case History #771 (2022) is the smallest of the three and the most typical of what a homeowner actually does. A Minnesota homeowner remodeling a house needed a radiator cover, found new ones expensive, and instead bought two decorative cast-iron grates at a secondhand store to build one. Both were rusty. He applied CorrVerter directly over the rust to passivate it, let it dry, and put a white finish topcoat over it. The grates became the front of a working radiator cover for a fraction of the cost of a new one.
| Case | Prep | Primer | Topcoat | Reported result |
|---|---|---|---|---|
| 1947 sedan, #769 (2008) | Scrape and degrease; no blasting | CorrVerter over remaining rust | VpCI-386 Black | No rust creep at fasteners after almost 14 years |
| Civil War cannon, #193 (1997) | Sandblast | VpCI-373 wash primer, 24-hour cure | VpCI-386 Black and Blue | 5 to 10 years outdoor protection |
| Cast-iron grates, #771 (2022) | None beyond loose rust | CorrVerter over rust | White finish coat | Decorative, in service indoors |
The rules that do not change for a hobby job
CorrVerter converts rust; it does not replace surface prep. Loose rust, scale, grease and oil come off first, to hand-tool clean: wire brush, scraper or wire wheel. Tight, adherent rust stays, and that is what the chelating agent binds into a hydrophobic, paintable layer. It is a ferrous-only product, so keep it off aluminum trim, galvanized hardware and brass. Apply it between 45 and 90 °F on a surface above the dew point, and do not let it freeze before it dries. One coat at 3 to 5 mils dry is the target, and it must be topcoated; the converter is a primer, not a finish. If you have blasted to clean metal, skip the CorrVerter and go to VpCI-373 or straight to 386, as the cannon shop did.
How to do this in your shop
- Degrease first. Scrape the heavy grease, then wash with an alkaline cleaner such as VpCI-414 and rinse. A Build-All or Magido parts washer handles loose parts; a bucket and brush handles the floor pan.
- Decide whether to blast. Parts that come off the car can go through a blast cabinet or a blast room and then be primed on clean metal. Anything still attached gets wire-brushed to hand-tool clean and converted.
- Brush or spray CorrVerter over the remaining rust at 3 to 5 mils dry, working it into seams and around fasteners. A gallon covers roughly 115 to 190 sq ft, which is more than enough for a car underbody.
- Let it cure, then topcoat with VpCI-386 Black at 1.5 to 3 mils dry. On blasted steel, or on any aluminum or galvanized piece, put down VpCI-373 at half a mil first and give it the 24 hours the cannon shop did.
- Follow the spray and brush settings in our coatings application guides; the same tip sizes and recoat windows apply whether the job is a truck fleet or one sedan.
- Check the bolt heads a year later. That is where rust creep shows first, and it is the test the 1947 sedan passed at 14 years.
Which Cortec products
- CorrVerter, 1 gallon: the restorer’s size; one-coat water-based rust converter primer for steel and cast iron that cannot be blasted.
- CorrVerter, 5-gallon pail: for a shop doing frames, trailers or structural repairs on a regular basis.
- VpCI-386 Black, 5-gallon pail: the black topcoat in both the sedan and cannon cases; water-based acrylic, direct to metal, rated for outside, uncovered service.
- VpCI-373 wash primer: the adhesion primer under 386 on blasted steel, aluminum, galvanized and plated parts.
We stock CorrVerter, VpCI-386 and VpCI-373 in Elmhurst, Illinois, and we sell them by the gallon and the pail to hobbyists as readily as to fleet shops. If you have a project car, a piece of farm iron or a monument that needs to stop rusting where it stands, contact us and we will match the primer and topcoat to the metal, or send a sample so you can try it on one panel first.
Sources: Cortec Case History #769 (2008), Cortec Case History #193 (1997), Cortec Case History #771 (2022); Cortec product data sheets for CorrVerter, VpCI-386 and VpCI-373.
