VCCA Home Forums TECHNICAL - GENERAL Body - Sheet Metal, Paint & Wood Keeping Bare Metal Rust Free

After responding in the 37-41 forum to the topic “Body Repair … Order of Work”, I was asked in a PM how I keep bare metal from flash rusting during the repair process and after blasting. I thought I would share my experience here rather than in a PM. I have been using this process since the mid 70’s with very good results however it’s labor intensive and probably not something that you would want to pay someone to do. Also, I buy large pieces of sheet metal in various gauges to keep on hand when I need to fabricate a repair panel. I use this same process to keep them from rusting as well. I’m sure there are other methods just as effective but here goes:

… Whenever I have clean bare metal that I want to stay rust free for weeks, months or even years I douse it with Dupont (now Axalta) Metal Prep 5717s diluted according to the directions on the label. I believe the product now comes ready to use rather than in concentrate form. The product description says it’s a non-flammable, phosphoric, acid-based conversion coating that produces a uniform zinc phosphate coating on steel and galvanized steel surfaces. The description goes on to say the zinc phosphate coating formed on the metal surface offers the best possible substrate for paint system adhesion and corrosion resistance. Some people have told me Ospho is the same thing but I’ve never tried it.

… Once the surface is thoroughly wet I simply wipe off the excess with a clean rag and use a heat source or compressed air to quickly dry it. Contrary to the directions in the TDS, I do not immediately rinse with water. When quickly dried, the resulting surface will look dull, powdery and stay rust free for a very long time. If the surface remains wet for too long, some minor blushing will occur that can be removed by rewetting and drying quicker. In addition I’ve found that scrubbing a wetted surface with scotch brite will remove minimal rust blushing that might have occurred after blasting. In this condition the surface is not ready for paint, it has too much dried material in it.

… When I’m ready to weld a seam, use body filler or prepare an entire surface for the initial coat of epoxy primer, I use a wire wheel on an air tool and or an orbital sander to remove the white residue that was left on the surface when the 5717s dried. Using an orbital sander with course paper gives the surface good tooth for adhesion and a wire wheel effectively cleans the residue from pits. The powdery residue comes off fairly easy but it’s important not to miss any areas.

… Although there are two schools of thought, I use body filler over bare metal that has been etched and properly cleaned rather than filler over primer. After I’m satisfied with the filled surface, I spray two coats of non-sanding epoxy primer, usually PPG DP 90LF as a foundation. I then go on to using the high build surfacers, a sealer and finally the top coats always sanded and buffed.

… As always, when finished, stand back with an adult beverage in hand and admire your progress. Hope this helps.

Mike

I have been using Ospho the same way for over 30 years. The key is wiping off when the surface is still damp so the light to white powdery coat does not form. If the surface is not dried fairly quickly a tacky surface forms that takes hours to days to dry and then has to be cleaned with paint thinner, sanded or metal prep reapplied. Rewetting and then wiping will remove it. I use clean paper towel to dry the surface or compressed air to blow it dry. As long as water does not drip or condense on the coated metal it will not rust for a long time. If I know it will be and extended time I spray with self-etching primer. It gives a longer lasting rust free surface but not as good as a good epoxy or urethane primer sealer.

This is also the way I have always done it but with the LOW VOC versions of the chemicals this method has become an issue.

My PPG Epoxy Primer data sheets say to NEVER apply the primer over etched or treated metal, even if it is sanded as it provides a failure point.
The data sheets also say to NEVER apply etching or metal treatments to SANDBLASTED metal since the sludge produced by the chemical reaction cannot be washed or wiped completely out of the depth of the pits and provides a failure point. For the same reason they do not want wax/grease remover used on sandblasted metal. "If it's sandblasted it is clean and ready for Epoxy Primer, just blow off with clean compressed air".

Sadly I can only buy the low VOC versions of the products here in Canada and it looks like most of the States are the same with maybe a couple states where the good stuff is allowed. I'm applying the PPG epoxy primer to sandblasted metal so it's a double negative for using a treatment like Ospho. It would make life so simple at this stage of the project but paint failure later really concerns me. All of the low VOC paint products seem to be less forgiving so I'm reluctant to stray from the Data Sheets.

Part of me does wonder if there is truly an incompatibility with the new low VOC products or whether some of it is propaganda to keep us from using some of these nasty chemicals and processes.

I have some 85% phosphoric acid from a local chemical supplier. Would this be effective to remove or treat rust? Should it be diluted?