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DuPont 226s conversion coating does not turn golden on a 6061-T6 spar

May 18, 2008

Q. Good evening,
After almost three years of post-Katrina stress, I am finally refinishing the spar on our former home, a 40 foot sailboat that was moored in beautiful, Bay Saint Louis, Mississippi. We took the "direct" hit from the storm (contrary to the reports from New Orleans) with 34 feet of tidal surge. While hundreds of vessels were lost, ours only traveled 1/2 mile and ended upright between two pines.
Because so many businesses and services were lost on the coast, I have been forced to become the sole contractor on the project learning many new skills and honing others.
The paint was stripped and the spar sanded with 80. Now, I want to replace the Alodine lost in the process. Now, I got to tell ya, I have the cleanest spar in south Mississippi! Not because of anything intentional, but I have now applied the DuPont 225s and 226S product twice to a 48 foot stick and it looks like I am going to have to take another swing at it. I just don't know where I might have messed up the process. So, I'll describe the process to date:
Strike one:
After sanding I washed and then "Paint Prepped" the substrate to remove any oils.
Applied 225S per instructions, then rinsed. Nice, even sheeting. I dried the substrate before applying the 226S. I used a rag to apply the 226, kept the process wet for 3-5 minutes; rinsed. Nothing happened.
After pondering what went wrong, I threw up my hands and contacted the folks at Van's aircraft web site. One of the members suggested that the 6061-T6 takes longer to accept the conversion so increase the time to 15 min. Most often, aircraft parts are placed into a bath of etch and Alodine, but I could not (would not) build a tub 48 feet long. So, I am using a brush to apply the 226S.
OK, I am optimistic, now.
Strike two:
Re-applied the 225. Pressed my wife into service to aid in rinsing. I had several streaks where the 225 exposure was greater prior to rinsing. I rescrubbed the spar with the 225 and a green Scotch-brite. The results were far better- no streaks. Let the product work for a minute or two.
Rinsed and keeping the area wet, I applied the 226 with a brush and kept each section wet for 15 min. When I rinsed, nothing. Dejected, I had a cold-one and went home.
That was Friday, today being Sunday, I decided that maybe, the color change just wasn't going to happen. So, I went down, with all the intent to apply the primer/sealers and be done with it. When I wiped the spar down, there was evidence of a little, not much, oxide on the cloth. So, the conversion was not complete! Is it possible to have a bad batch of 226? How would you tell?
Before I go for another swing and a miss, I need help from the gurus here at finishing!
All suggestions, insights, and admonishments will be taken as a student, because this is, most certainly, a learning experience.

All the best,

Kevin McGreevy
conscripted contractor/student/hobbyist - Gulfport, MS, USA

Q. Ted,
None of the readers seem to have any insight or inclination into this situation. Would you have any recommendations. My inclination is to acid prep wash, dry and shoot with strontium zinc primer. What do you think?

All the best,

Kevin McGreevy
- Gulfport, MS, USA
June 18, 2008

A. Hi, Kevin. Chromate conversion coatings do not have a shelf life limitation (for non mil-spec work). Still, your suspicion that this batch might be bad (mislabeled somewhere along the line) appeals to me. If you don't have a lot left over you might try a different brand like Alumiprep 33 [on eBay or Amazon affil links] and Alodine 1201 [on eBay or Amazon affil links] just to eliminate the chance of a second bad batch.

Also, rather than doing a 40 foot mast, I'd try a 6 inch section of the mast first :-)


Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey
June 18, 2008

Q. For Aluminum that we need to powder coat, will Dupont 225s/226s work well or is there another product which cleans/prep's for powder coating of aluminum work better and stand up to the abuse of road debris (sand blasting from the front of a vehicle). Thanks.

Charlie McNall
- Waklkerton, Ontario, Canada
July 4, 2013

A. Yes, you can get a bad batch. We were badly hurt when one of the biggest names gave us 3 containers in a row of bad material. They finally owned up to their retained sample being bad.

Spray or brush on chem film is not the same as a "dip" in the solution. It may be a different concentration or even a different product that they make.

I would suspect that at 15 min, that the pH is bad off. Assuming that the chrome concentration is correct.

James Watts
- Navarre, Florida
July 5, 2013

Do not let the surface dry after 225S rinse. Apply 226S while the surface is wet and clean. 2 to 5 minutes of 226S is sufficient.

Vijay Merchant
Omni Metal Finishing, Inc - Mission Viejo, California USA
September 8, 2016

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