Issue link: https://www.e-digitaleditions.com/i/1014893
Sealing seams on parts before powder coating Q I'm trying to find a caulking sealant that is compatible w i t h t h e t e m p e r a t u r e s achieved during the cure cycle in our powder coating line. We make newspa- per and magazine presses and are cur- rently powder coating all of our sheet metal doors, guards, enclosures, and so on. We wet painted for years before converting to powder. The vendors used to caulk a lot of the seams to sat- isfy a watertight requirement while reducing the amount of weld and resul- tant distortion. Now, however, we're having a problem finding a caulking that can be applied to the bare metal, withstand more than 370°F in our ovens, and bond to the powder coating. Is there a product that you can recom- mend? Thanks, W.S., Dover, N.H. A Have you looked at an RTV nonsilicone material? The automotive industry, among other industries, uses sealants around various parts of car bodies. These seal- ants can withstand the temperatures you are using. —G.T. Believing what you see Q Like any shop, we do some recoats when the first coat is rejected, be it some pickup or light paint. My question is this: When we do recoats, we handle it with gloves, sand it, blow off the piece, and wipe with isopropyl alco- hol. When we try to repaint the piece, it appears to have streaks where the powder won't stick (like bare spots where it was touched by hands with latex gloves or wiped). I have had this problem before, and it stopped when I heated the product up after handling and then waited for it to cool before powder coating it. I've tried to suggest this to my supervisor, but he thinks it's something else and not from handling the piece. But I can see clear finger marks, and I even wrote num- bers on the product with the gloves, and the paint wouldn't sit where I did this. I'm just wondering what this is and if I'm right in saying to heat it up or rinse it in cold water and dry it or let it sit after handling for a period of time to prevent this from happening. J.M., Langley, B.C. A You know the old saying: "Believe nothing that you hear and only half of what you see." This proverb applies here, as well. However, it doesn't say ignore what you see. To that end, if you see handprints or fingerprints under the coating, then they're there no matter what your boss says. Gloves often give people a false sense of security. The type of gloves and the frequency between changing them are two factors that affect the protection you obtain by using them in the first place. The best gloves are lint-free cot- ton gloves (preferably white in color). Change the gloves often, especially when the white color is stained or dirty. Lint-free cotton is comfortable and ensures that the operator doesn't have an allergic reaction or deposit lint onto the surface. Latex gloves have powder to keep them from sticking to each other and to make them easier to put on your hands. This powder can be a source of contamina- tion. Besides, latex gloves are very un- comfortable to wear, especially when it's warm and your hands are sweaty. Recoats need to be handled differently than virgin parts. The surface must be completely cleaned after the defect has been removed (by sanding or strip- ping). Often, the entire surface needs to be scuff-sanded to ensure proper ad- hesion to the topcoat. All the dust must be removed and the surface completely cleaned before applying the next coat. Don't use tack-rags, as they can often leave a residue on the part surface that will prevent the powder coating from adhering to the part (not a problem with liquid coatings, as the solvent will penetrate this contaminant). Denatured alcohol is good, if you change the rag and solution fre- quently. Using a dirty rag and contam- inated alcohol will just reapply the soils onto the part. Using the alcohol di- rectly from the can on a clean rag is best, changing the rag areas often to ensure cleanliness. —N.L. Powder coating passivated materials Q Will powder coating adhe- sion be compromised when coating on passivated mate- rial, such as galvanized sheet steel? J.S., York, Pa. A No more so than with any other coating. Galvanized metal like other treated met- als, such as tin plate and even Corten steel, are, by design, self-protecting. Then we geniuses try to second-guess those manufacturers by applying paint to the metal, so that it looks nice. But if POWDER COATING, August 2018 27 George Trigg GRT Engineering Nick Liberto Powder Coating Consultants Questions & Answers