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  #1  
Old 10-11-2018, 06:47 PM
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Strikefinder Strikefinder is offline
 
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Location: Hebron, CT
Posts: 40
Default Dimpling after SEM Primer

Hi all,

I worked my way through the tail using the NAPA 7220, and decided pretty early on that to get the best performance of the primer, I would match drill, scuff, dimple, then prime...and for the most part that was achievable with only minor modifications to the order of the steps in assembly. I wasn't very happy with the 7220, so I bought a few cans of SEM, and like its application much better--I find its adhesion and coverage much better, so I decided that I would use that for the wings.

I got as far as the rear spar before it became apparent that, if I was following the directions, I'd have to prime the rear spar components and rivet them together prior to match drilling for skins, which meant I'd be dimpling after priming the rear spar. I prepped the rear spar, primed it with SEM, and let it cure for over a week (not intentionally, just was out of town on a trip).

Just to try it out, I reamed one hole and tried to dimple it using my pneumatic squeezer, and the primer just flaked off around where the dimple die contacted the spar. I can scratch it pretty aggressively on the web without failure, so I was hoping the flange would survive the dimpling process, but it looks like once I get the skins match drilled and I go to dimple them, it's all going to flake off.

So, a few questions:

1. Other than dimpling and then reapplying a coat of SEM afterward, is there anything else I can/should do with the parts that are already primed?

2. It seems like a fair amount of the wing substructure (ribs, etc.) is primed, then dimpled after match drilling the skins. Has anybody changed the order so that dimpling could be done before priming? I'm reluctant to deviate from plans too much--I've tried a few times to do things out of order, and usually after I screw it up I figure out why it was suggested I do it in the order suggested.

3. For prep, I've been doing a pretty good scrub with maroon scotchbrite on all surfaces, followed by an acetone wipe, let it dry for a good 15 minutes, and then spraying the SEM with a few light coats. I use gloves to ensure no contamination occurs after wiping down with the acetone or during the painting process. I hate working with chemicals if I don't have to, so while I've considered doing PreKote or something similar, I would prefer to avoid making the prep any more complicated than it already is.

4. I've thought about switching to EkoPoxy, but I figure I've got about $600 to invest in equipment (paint gun, paint+shipping, mixing cups, etc.), and I'm intimidated by the prep and clean up. With two small kids, most of my work sessions come in 15-45 minute bursts, and so doing something that requires 40 minutes of setup/cleanup (or more) means I lose a lot of opportunities that I can currently use to spray a few parts or drill a few holes. Is it night and day difference to go to a product like this?

Would appreciate any advice!
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  #2  
Old 10-12-2018, 08:00 AM
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Strikefinder Strikefinder is offline
 
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Location: Hebron, CT
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Default

After a lot more searching and research, I'm wondering if maybe I'm applying too thick? It seems like when I apply the first coat, it's fairly transparent in spots, so I've gone over it on subsequent passes and tried to ensure everything is at least "filled in". Is that a source of my problems?

The SEM instructions that say to "achieve hiding" isn't exactly clear to me, and I'm afraid of leaving areas open to corrosion if I don't cover it enough...that said, it doesn't help if it flakes off.

Humidity is also a big problem where I am, but I don't know what to do about controlling that. Seems like it's always raining on paint days to begin with, but even the dry days are pretty steamy near my home.
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  #3  
Old 10-12-2018, 09:53 AM
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wirejock wirejock is offline
 
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Location: Estes Park, CO
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Default Prep

I'm one who dimples before priming. Priming is my last step before assembly.
Usually flaking of is a sign of inadequate prep. Paint doesn't like the aluminum oxide layer. You're scuffing should remove it but maybe something is contaminating the surface. If you want to eliminate chemicals, try a wash step instead of acetone. Use gray scotchbrite and Bon Ami cleanser. Scrub thoroughly, rinse well, dry and prime.
Keep thin on the tail. All that primer adds weight. Spray a little in a can and use a q-tip to repair flaked areas.
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Larry Larson
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  #4  
Old 10-12-2018, 12:36 PM
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N804RV N804RV is offline
 
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Location: Mount Vernon, Wa
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Default

I've dimpled after priming with mixed success. Usually its fine. But, sometimes, if I do it too soon, I'll damage the primer coat. Then, I'll scuff lightly and spot prime. Haven't had any problems.

----But, having done the whole milspec corrosion prevention and control thing professionally, I'm a firm believer in contributing the minimum to the primer wars where GA kit airplanes are concerned.
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  #5  
Old 10-12-2018, 06:41 PM
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Strikefinder Strikefinder is offline
 
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So, just to be clear, you gentlemen didn't prime and rivet the aft ribs to the spars prior to match drilling the skins and dimpling? My gut was to skip the priming and riveting the front and rear spars to the aft ribs, match drill the skins, disassemble everything, dimple, prime, then reassemble and rivet. I'm only reluctant to do so because I wasn't sure if there was a good reason it stated to do it the other way (prime, rivet ribs, then match drill skins) in the plans.
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  #6  
Old 10-12-2018, 07:07 PM
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wirejock wirejock is offline
 
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Default Huh?

Quote:
Originally Posted by Strikefinder View Post
So, just to be clear, you gentlemen didn't prime and rivet the aft ribs to the spars prior to match drilling the skins and dimpling? My gut was to skip the priming and riveting the front and rear spars to the aft ribs, match drill the skins, disassemble everything, dimple, prime, then reassemble and rivet. I'm only reluctant to do so because I wasn't sure if there was a good reason it stated to do it the other way (prime, rivet ribs, then match drill skins) in the plans.
Sort of. There were a few parts where I dimpled after the part was riveted to another part. No biggie. None of my P60G2 primer came off. Wherever possible everything was assembled, match drilled, disassembled, deburred, scuffed, dimpled, scrubbed with BonAmi, rinsed, dried, primed then reassembled. Interior parts were also sprayed with interior paint within a 30 minute window, then rubbed smooth with a claybar prior to reassembly so the rivet heads would show. Yea. Tons of work but it looks awesome.
Sometimes the instructions direct you to rivet. Nothing wrong with it. You can choose how you want to do it.
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Larry Larson
Estes Park, CO
http://wirejockrv7a.blogspot.com
wirejock at yahoo dot com
Donated 12/01/2017. Plus a little extra.
RV-7A #73391, N511RV reserved (1,800+ hours)
HS SB, empennage, tanks, wings, fuse, working finishing kit
Disclaimer
I cannot be, nor will I be, held responsible if you try to do the same things I do and it does not work and/or causes you loss, injury, or even death in the process.
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