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I Knew Better...

1001001

Well Known Member
...but I Wasn't Thinking

I think I know what happened here, but can anyone tell me and help confirm or deny my suspicion?

20160626_220112-768x432.jpg


http://www.brunbergs.net/?p=564
 
Let me guess.....
You didn't acetone or scotchbrite before using a self-etching primer?
Lorne

I used both acetone and an Alumiprep etch prior to the selt etching primer, same basic process I have used on all my prior primed parts.
 
I've had that cracking/flaking happen when I applied way too much paint (or primer) to the surface.
 
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My guess is moisture. Compressor filter failed/bypassed or moving from air conditioning (cold part) to warm humid outside?

Reflex
 
My guess is moisture. Compressor filter failed/bypassed or moving from air conditioning (cold part) to warm humid outside?

Reflex

That is my thought. I spent too much time prepping and by the time I was ready to spray (rattle cans), it was dusk and the temperature plummeted and the relative humidity skyrocketed. I feared it would be a problem but I underestimated just how much moisture was in the air.

The only other two differences in my process this time was I wiped the Alumiprep on rather than soaking the ribs, and rinsed with potable water instead of distilled.
 
I feel your pain, same thing happened to me. Probably the largest batch of parts I primed at one time during the entire build. Like you, evening was coming on, I moved all the parts outside to prime. Didn't realize how much evening moisture was already in the air. Looked great right after I primed, went back later to check, pretty discouraging. Just one of those small setbacks, we don't plan on. Clean em up, reprime and build on! It's all worth it!
 
Moisture. I had a rudder hung on a clothesline and shot it with green primate right at dusk. Temp dropped, humidity spiked, and all the green cracked and fell off like yours. Next morning I re-prepped and shot the same primate and it was perfect. Then I topped it with Alumi-grip. Perfect. Then the rope broke and it fell in the dirt.
 
Moisture. I had a rudder hung on a clothesline and shot it with green primate right at dusk. Temp dropped, humidity spiked, and all the green cracked and fell off like yours. Next morning I re-prepped and shot the same primate and it was perfect. Then I topped it with Alumi-grip. Perfect. Then the rope broke and it fell in the dirt.

Aaaauuugh!
 
why etch before using a self-etching primer?

Because I believe it does a better job of surface preparation than scuffing alone, and I suspect that the etchant in the self etching primer is perhaps not as aggressive as the Alumiprep?

I suppose I could resolve the question and possibly save myself some time and effort by making test coupons with both processes and doing pull tests.
 
Peeling primer

Yup
Happens when you etch the metal, thinking it will help the self etch product grip better.
Years ago this worked, but my experience is that the modern self etch primers do not like pre etched metal. I have found that when you remove the mess, the second try will adhere well. Scotchbrite, clean with acetone or similar, spray to coat.
Rattle can products I have used don't mention pre etch on their directions, for a reason.
 
Yup
Happens when you etch the metal, thinking it will help the self etch product grip better.
Years ago this worked, but my experience is that the modern self etch primers do not like pre etched metal. I have found that when you remove the mess, the second try will adhere well. Scotchbrite, clean with acetone or similar, spray to coat.
Rattle can products I have used don't mention pre etch on their directions, for a reason.

This has not been my experience. I have primed a number of parts using the Alumiprep etch before SEM self etching rattle can primer, and all others have been just fine.
 
I had a similar issue...

With my tail section side skins. Cool humid day - I was keeping my Stewart Systems primer inside the warm house, aluminum was nice and cold and it was damp. It looked like someone had sneezed primer on the skins; everything just slowly slid down into a big puddle. :eek: Too embarrassed to make any pictures. Fortunately I could just hose it off and start over when the weather improved.
 
Type of primer?

I am not sure what type of primer you used. I used a two part expoxy primer and would suggest that you might have water in your lines as said before.

Or if a two part primer i make SURE that it is exactly even by measure by Weight! Being unequal in weight should only affect drying but if you were way off it might have shrunk too much

Sorry i am only guess which I am sure you have done lots of at this point.
 
Sponge brush self etch primer

I found spray on primer too time consuming and expensive. After I complete my de-burring, I smooth the surface with a scotch brite pad, then clean with some diluted degreaser detergent in a spray bottle and wipe clean. Then I mix a small amount of DuPont Veriprime 615S mixed thin, and brush on with a sponge brush. It seems to be quick, easy, thin and light. And it also seems to provide scratch protection during the build similar to the plastic film that some builders leave on their exterior skins.

2qw3fic.jpg
 
Before priming all your parts with any primer do the 100LL test. Wipe some Av gas on the primed parts, keep them a bit moist and then wipe with a towel. Most rattle can primers will fail this test.
Cheap, easy primer is worse then no primer.
 
I've seen this

I get this same problem occasionally, but it is always in very small areas - usually in a corner or radius of a formed part. Never as bad as your photograph shows though.

I wash/scotchbrite with detergent, etch with metal prep, wipe with lacquer thinner, and then prime with SEM rattle can.

I've never figured out why this happens. My fix is to lightly scrub the area with a scotchbrite pad and lacquer thinner and re spray. That always works.
 
I've run into this problem with the SEM Self-Etching rattle can. I found the consistency of the paint wasn't always great and it was prone to drips out of the nozzle. These drops would dry into flaking sections. I think this was happening more often if it was cooler and if I didn't shake the can enough.

I can make this problem happen easily now just by leaving a little moisture on the surface. If it isn't completely dry, the paint will bubble on contact. I catch this right away now, wipe away the bubble and re-spray. For the sake of disclosure, my current prep scheme is acetone wipe (only as needed), soapy water spray, scotchbrite scrub, clean water spray and wipe off with a lint free shop towel. I do this all in one sitting, painting within a half hour.
 
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