Corrosion Protection of Frames
Scott Selbach – Feb 1999
Due to my expertise in the field of the corrosion protection of steel substrates, several people on the list suggested I write this thesis on the preservation of Jaguar frames. This is a very cursory dissertation, but should provide the basics anyway. If this topic interests you, read on.
The coating of any steel surface for corrosion protection can be divided into three important areas: 1) surface preparation, 2) coating selection, and 3) coating application. The weak link theory applies here and each of these aspects has to be done properly.
There are a number of ways to prepare a steel substrate, depending on the intended use, desired result, and budget. The basic methods of preparation are: 1) solvent cleaning (includes washing), 2) abrasive blasting, 3) hand/power tool cleaning, and 4) chemical cleaning. All surfaces to be prepared must be washed and solvent cleaned. We can fly to the moon but cannot successfully paint over grease, oil and salts. Washing with a solution of hot water and a strong detergent such as TSP (tri-sodium phosphate), Dirtex, Soilax, etc. will remove road salts and general dirt. The solvent cleaning with MEK (methyl ethyl ketone), toluol, xylol, or wax and grease remover will remove grease and oil deposits. Most preparation methods do not remove this and actually force it into the steel surface. Remove the contaminants first, then de-rust the steel.
The best prep for maximum long-term corrosion protection is abrasive blasting. This removes existing corrosion and prepares the surface to accept a corrosion resistive coating system. The abrasive can be any one of a number of media including silica sand, coal slag (Black Beauty), plastic, steel, water, ice, aluminum oxide, glass, sodium bicarbonate, sponge (believe it), corn cobs, walnut shells, etc, etc. Silica sand and Black Beauty are the most widely used as their characteristics best suit our needs. The results of an abrasive blast are measured by the cleanliness and profile. Cleanliness refers to how much of the corrosion in the steel is removed and is important to the longevity of the corrosion protection. Profile is the texture of the blasted surface and is important to the adhesion of the primer.
Hand and or power tool cleaning is simply using a wire brush or other such device by hand or powered. It will not get the surface as rust free and provides little or no profile. The roughness left after using a grinder is not an acceptable profile. You may have cleaned a steel surface with a powered wire brush in the past and thought that it was rust free, but if you were to compare that to a “white metal” blast, you would see a huge difference. A lot of corrosion is left in the steel pores which will come back to visit you later. The attractiveness of hand/power tool cleaning is that it is inexpensive, can be done in the home setting with little tool investment and expertise, but it does compromise corrosion protection and longevity. Chemical dipping can also be done but I think is better left to the body panels. Frames have a lot of inaccessible areas that are difficult to purgue the chemicals out of and that can cause problems later.
With all of this said, let’s get to a frame. The outside of the frame is best prepared by abrasive blasting to a silvery steel look with a profile that feels like medium sandpaper. Don’t do this inside your garage or you’ll get spent abrasive everywhere, and I mean everywhere. The abrasive can be gotten out of the inside of the frame by rotating and tipping it combined with compressed air blowing it out. That abrasive that is left in the frame can stay there, not enough to worry about and non damaging anyway. This is not a clean process, but if done with a little precaution, it doesn’t have to be terrible. Properly suit up in a Tyvek suit, taped at the wrists, ankles and neck and wear a good hood and gloves. Always try to paint a surface soon after preparation to prevent rust-back which can happen very quickly, especially in humid environments. The rule of thumb in industrial applications is to apply primer within 24 hours of blasting.
The inside of a frame is a different situation. Inaccessibility prohibits blasting and hand/power tool cleaning. Truthfully, there aren’t many good options. Chemical dipping is about the only thing that will get to all the inaccessible areas inside the frame but this comes at a price. Dippers neutralize the aggressive de-rusting acids, but as mentioned before not all the solution can be gotten out leaving them in the frame and they tend to keep eating away at the surfaces. I wouldn’t dip anything that I wasn’t absolutely sure I could purge 100%. You also can’t use conventional paints inside the frame. They won’t adhere to the poorly cleaned surfaces and you can’t get them in every nook and cranny without pouring them in. If you do that, though, then the coatings will build up somewhere, get too thick, crack, fall off and then you have no corrosion protection. For my money, unless the frame is really badly rusted, I would get as much of the crap out of the inside of the frame as possible by water and air power and poking around and apply a good thick coat of Waxoyl. This stuff will wet into the dirty, rusty surfaces, passivate the steel, and you can apply as much as you need to get it to flow all around the inside the frame without worrying about how thick it gets. Low tech, but awfully effective.
Most of the work is done; surface prep is the tough part. There are many different generic classes of coatings. For corrosion protection, you can’t beat zinc primers (they will even outlast a galvanized surface), but epoxies offer the best combination of protection and ease of application. Over a blasted surface, apply two coats of a good epoxy primer. McMaster-Carr or Grainger will have a Rust-Oleum product, or your local auto paint store will have products like PPG’s DP40 or DuPont’s Corlar primers. They are all two component products meaning there is an “A” and a “B” that have to be mixed thoroughly together. Once mixed there is a finite potlife that you have to use it within or it goes bad. Once on the surface, there is also a finite recoat window that you have to apply the topcoat within. Read the label directions entirely and follow them to the letter. After the epoxy primer(s), apply one coat of a urethane finish coat (like DuPont Imron) and the frame will look great. Imron is a high gloss but other products are available in lower sheens, if desired. Urethanes are also two component, so again, read the directions.
Over a hand/power tool cleaned surface, there are coatings the will offer decent protection over little or no profile and with some existing corrosion in the steel. POR15 and Miracle Paint are coatings for this condition, as are surface tolerant epoxy mastics. STEM’s will outperform POR15 or Miracle Paint and in an aggressive chemical exposures this difference would be big, but in this mild exposure, the difference in performance may not be a big deal. STEM’s can be effectively applied directly over tight rust (no loose or flaky rust) with applications known to last 12-15 years without evidence of re-rusting. POR15 or Miracle Paint will probably last 7-10 years, maybe more, maybe less, hard to predict (various factors such as – rural Utah is a lot different exposure than coastal Florida, miles driven, etc.)
Some people mention galvanizing and powder coating of frames. Both are coating options, albeit expensive ones. Galvanizing is basically permanent (on a car frame, anyway) and can be painted over. The frequent failure of painting galvanized surfaces is due to lack of knowledge, not any resistance of galvanizing to be painted. Powder coating is a process left to an expert with an elaborate facility set-up and is available in about any color and sheen. Touch up of the inevitable ding is more difficult but then powder coatings are more ding resistant. Only you can decide if you want to go to the extent and cost of either of these processes. Both are excellent, but this discussion has been aimed more at the do-it-yourselfer.
With surface prep being 65% of the job, coating selection being 25% of the job, there isn’t much leftover for application. This is the fun part though, that transforms the frame into a well protected and beautiful piece. About all I can say about application is to read the label directions of the coating you chose. These products were designed with specific application parameters. Many of these coatings were designed for spray application, and if you’ve gone this far, you’ll be a lot happier with the sprayed finish. A sprayed finish can mean the difference between looking like a professional versus an amateur result. Trust the coating’s designer and follow his instructions, more is not always better. Avoid that temptation by putting the coatings on at the recommended thicknesses. Coating thickness is measured in mils (1 mil= .001″) and if you exceed the designed millage by too much then drying, running, and cracking can become a problem. Too little and you haven’t got the protection you may need.
Oh, by the way, I mentioned before about putting Waxoyl inside the frame. Paint absolutely doesn’t stick to wax, so finish painting the outside of the frame first, then apply the Waxoyl inside, then clean your sloppiness from the painted outside surfaces with mineral spirits.
I hope this covers the subject and has helped in some way. If all steel coating projects were this easy, however, I wouldn’t get paid enough to keep my car collection in Castrol, so there are many variables that can affect the selection of prep method, coating, etc. Private email me at firstname.lastname@example.org if you have any further questions. I would be glad to recommend a coating process customized to your conditions of capability, intent and budget.
’53 XK120M FHC
’60 Morris Minor 1000
’54 Ford F100
’76 BMW 2002