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POWDER COATING, November 2018 27 By popular demand, we're bringing you a series of past pretreatment problem columns, representing the best of Brad B. Gruss, our colum- nist for more than 20 years, who has now retired as president of the con- sulting firm, Pretreatment & Pro- cess Consulting LLC. Brad special- ized in training, troubleshooting, and independent line audits for pretreatment processes. In his more than 30 years in the industry, he made numerous presentations on pretreatment for powder coating. Prepping parts by soda blasting Problem This question pertains to soda blasting and powder coating. If I soda blast a part, what's the proper procedure to prepare the part after that? Soda blasting will leave a residue, which is supposed to keep the part from rusting inside with low humidity for up to 60 days. at being said, will this residue affect the adhesion of the powder coating? If so, what should be done to remove the soda residue be- fore powder coating? Have there been any reported problems from using soda blasting in the past as a means of preparing a part? R.D., Rockford, Ill. Solution is is unique, but a great question many may not have consid- ered. The key here is the statement that the remaining soda blast film will prevent oxidation in controlled-hu- midity situations and indoor storage for up to 60 days. Well, that's gener- ally true for any remaining alkaline film. ere are two issues to consider. 1. Is the remaining film loosely ad- hered to the substrate, which may cause a loss of dry-film adhesion? Any loss of dry-film adhesion will have a corresponding reduction in humidity or salt-spray exposure. 2. Most (but not all) coatings prefer to sit upon a slightly acidic film with low remaining solids, or total dissolved solids (TDS). The pH should be at 5.5 to 6.8 with TDS less than 150 (10-50 preferred). I've seen several cases in which exces- sive alkalinity, especially hydroxides, will be very detrimental to coatings with polyester resins. My guess is that the remaining soda ash or residue isn't increasing performance in adhesion and corrosion protection. To prove or disprove this theory, a simple test can be done. Make 10 or 20 panels from the substrate that is being blasted. Process a set of panels with the blast residue, powder coat them, and cure as normal. Set these aside. Take a second set of panels, and after blasting, pretreat in either a five- stage process (preferred) or a three- stage iron phosphate system. Coat and cure these in the same manner. Care- fully apply the same mil thickness of coating on both sets, or process enough panels so that you can cull or eliminate panels with either high coat- ing thickness or low coating thickness. After scribing, place both sets of pan- els in either humidity or salt-spray en- vironments according to ASTM B-117, and wait for the results. My guess is that the post-pretreated panels will be far superior to the nontreated panels. I hope this will prove to pro- vide the correct answer. Prepping polished aluminum rims Problem A customer brought me a set of polished aluminum rims to clear coat, and I can't decide how I should prep them. An acid-based degreaser may cloud the polished areas, while anything less may leave the polishing compound in the metal and cause im- perfections or outgassing. What do you recommend? I always preheat any cast metal to 450°F for an hour before coating to prevent outgassing. J.M., Ossian, Ind. Solution You're correct. Polished aluminum is very sensitive to any alkaline (especially caustic-containing) a n d a n y a c i d c l e a n e r o r c l e a n e r phosphate. You'll probably dull the high luster. You do have to remove the buffing compound to achieve good adhesion. Try a solvent to remove the buffing compound, and then carefully wash in a (very) mild detergent with a near neutral pH. en, city rinse and finally a DI or RO rinse to prevent spotting. Hope this helps. PC Editor's note For further reading, see articles under the Surface preparation heading in the "Index to Articles and Authors 1990- 2017" Reference and Buyer's Resource Issue, Powder Coating, vol. 28, no. 6 (December 2017), or search the Article Archive at www.pcoating.com. All arti- cles listed in the archive are available for free download to registered users. Brad B. Gruss Pretreatment & Process Consulting LLC BEST OF PRETREATMENT PROBLEM SOLVER