- 04 December 2006 -
Painting Problem Solver: Cold Rolled Steel
By Ron Joseph, Organic Coatings Editor
In this month's Editor's Choice section, Ron Joseph, Metal Finishing's Organic Editor, delves into the tricky situations regarding the pre-treatment and painting of steel—among other related issues.
Q: We have just started a new business in which we weld much better than we paint. We are having a terrible time getting our standard Rust-Oleum paint to stick well to new cold-rolled steel. What is the best way to prep the new steel, and are there any tips for thinning the primer and thinning the paint for use in an HVLP paint gun? Right now we are painting at too low a temperature. Is that the biggest cause of our problem?
A: Thanks for your question, but unfortunately I can't give you a simple answer. Surface preparation varies from the least expensive to the most sophisticated, depending on what is being painted, the volume of steel that must be treated on a daily basis, space considerations, temperature and humidity of the paint shop, and much more. Similarly, your questions about the spray guns and paint thinning are dependent on your state and local regulations.
Depending on what quality you are looking for in the end result, here is a list of methods in increasing level of sophistication and cost:
- The simplest and least-expensive method is to degrease the metal with an aqueous degreaser.
- Moving up from there, you can degrease and apply a wash primer.
- Three-stage iron phosphating comprises a degreaser/phosphate in one stage, followed by a water rinse and a seal rinse.
- Five-stage iron phosphating comprises: (1) a dedicated degreaser, (2) water rinse, (3) iron phosphate, (4) water rinse, and (5) DI water seal rinse.
- Five-stage zinc phosphating comprises: (1) a dedicated degreaser, (2) water rinse, (3) zinc phosphate, (4) water rinse, and (5) DI water seal rinse.
- Even the options provided above can be outdone by adding additional rinse stages to further increase the performance of the final coating finish.
OLIVE DRAB PAINT
Q: I have a 1950 M-38 Jeep that I want to have painted. Apparently, I need a mixing code for the paint shop to match the Olive Drab army paint for the vehicle. Can you give me such a code or tell me where I might find it?
A: The following Web site gives you various shades of olive drab as it evolved during and after WWII: www.olive-drab.com/od_mvg_camo.php3. Select the version of olive drab that you wish to match. All of the color numbers, such as #34087, #33070, etc., refer to Federal Standard 595a color cards. Most paint companies have copies of the Fed. Std. 595a and some will be willing to match the color for you.
IRON PHOSPHATE AS A FRICTION SURFACE
Q: We apply a phosphate coating to the inside and outside of steel tubes. The external surface receives a PTFE electrocoating, but we need to insert a bearing on the inside of the tube. The problem is that when we press the bearing into the tube, the friction force between the bearing and the steel is at our upper specification limit. To reduce the friction, can we apply a dry film lubricant over the phosphate?
A: The phosphate coating is microscopically thin and if you are using an iron phosphate, then it is thinner than a zinc phosphate. Without conducting laboratory tests in a mechanical test laboratory, I cannot confirm that the application of a dry film lubricant is a good solution.
A common problem with friction-fit surfaces is that, over a period of time, the coating might degrade or shrink, in which case the bearing can come loose. I don’t know that this is the case with phosphate coatings, but by applying yet another coating (the dry film lubricant) over the phosphate, in my opinion, you complicate the issue and open yourself to more vulnerability.
Rather, you might consider removing the phosphate on the inside or plugging the end of the tubes before they go into the phosphate bath. I assume you have already considered the latter. Acidic solutions are available for dissolving the phosphate, and your chemical supplier can provide you with the appropriate formulation.
If you decide to pursue your idea of applying a dry film lubricant, I strongly suggest that you first conduct extensive laboratory tests under a range of environmental conditions to satisfy yourself that the bearing will not loosen with time.
FIRE PROTECTION OF STEEL
Q: Bob Pliskin has kindly answered the following question: I need to specify a fire protective coating with an architectural finish for an exterior, load-bearing steel facade. The facade will be prefabricated off-site in sections for shipping and site erection. The individual members are built up from plate approximately 1 5/8 in. x 12 in. and can be welded or bolt connected on site.
A: Typically, powder coatings are applied as architectural finishes for exterior use on metals. They are applied at temperatures that would activate most intumescent fire-resistive coatings, so no one has developed a fire-resistive/refractory coating using powder coats. Fire-resistive intumescent paints that are acrylic base do not adhere well over time to metals and do not stand up to external weathering on steel. Epoxy-based, fire-resistive coatings adhere well, are anti-corrosive, and are most effective in the thin-film category, but all epoxies yellow under UV exposure. The solution is to use an epoxy-based active refractory/fire-resistive coating and a urethane-based finish coat that will stand up well to weathering and not affect the performance of the base coat in a fire. Both the fire-resistive coating and the urethane top coat can be touched up on site where powder coatings cannot be easily repaired.
CONTAMINATION OF PHOSPHATE PRETREATMENT SYSTEM
Q: We have a new product coming in, and it has been soldered with a copper material. My question is, will it contaminate my pretreat system?
A: In my opinion, the copper is unlikely to contaminate the pretreatment system, but I don’t know for sure. I base this on my understanding that copper is a noble metal that does not dissolve easily in a mild acidic solution, such as one finds in a phosphate pretreatment system. In addition, I don’t know what the solder will do.
If I were in your situation, I would fill a beaker with the pretreatment chemicals and then immerse the copper/solder assembly for a few hours to see if the bath is contaminated. The major component of solder is tin (Sn) but small quantities of lead (Pb), copper (Cu), and/or silver (Ag) might also be present. After you have soaked the soldered assembly into the pretreatment bath for a few hours, send a sample of the liquid to an analytical chemistry laboratory and ask the lab to analyze the solution for copper, lead, tin, and silver.
If the bath proves to be contaminated, immerse samples of your other parts into the pretreatment bath and determine if you see any degradation in quality of the pretreatment finish.
Experimentation is the best way to determine the risk you run.
Ron Joseph is acoating consultant in San Jose, Calif. You can e-mail questions to rjoseph@exponent.com.
