The portion on the top is the scuffed part and the bottom half is the "cleaned only" part. There's no doubt that the scuffing makes a significant difference in the adhesion. That being said, I did some very aggressive scratching. Yes, the surface that was not scuffed definitely lifted easier compared to the other surface, but it still took a pretty significant amount of abuse. Once the primer came up, it did pull up in bigger chunks, as expected. I didn't take a picture, but I also tested how easily each side came up using acetone. EkoPrime is not solvent resistant, but the scuffed side still took a decent amount of rubbing with acetone to clean the surface. The cleaned only side softened up a lot faster and lifted in sheets.
My verdict from this test and the one I did before it a few months ago: I used to do pretty serious manual scuffing while acid etching. At least for EkoPrime, that's overkill. The acid etch alone (just using it to clean the surface, but not manual scuffing at the same time) is very nearly on par with manual scuffing, but I'd give the manual scuff a very slight lead if I had to choose. The cleaned but not scuffed parts surprised me by how well the primer still stuck. I would not take that approach on parts that are going to get some abuse, but for anything that just needs coverage and will be buttoned up without getting beat up, I wouldn't be concerned in the slightest about spraying EkoPrime directly on the cleaned surface with no scuffing. It performs far better than my self etching spray can primer does.
Given all of this, my process is going to change a bit since it sure looks like I was going overboard in prepping my surfaces. By scuffing along with acid etch, I was doing what is undoubtedly the best approach, but in reality, I don't think what's "best" is anywhere near worth the extra effort! It's like one of my old profs used to say, "if you get a 90% on one of my tests, it means you're ahead of the curve and going to be a very knowledgeable doctor. If you get a 95%, it means you studied twice as hard to learn a few bizarre facts that will never matter in the real world. Decide what matters to you, sleep or perfection." I always studied for the 95, but I think age has caught up with me. The added sweat and irritation to make the primer bullet proof will never actually matter in reality.
I really dislike the mess and process of acid etch/detergent because of the steps and time involved to etch and scuff, rinse, dry, etc. For most pieces, I'm no longer going to use the acid etch. I will lightly scuff, wipe down with acetone, and shoot primer. My manual scuffing is going to change as well. I really went at it before, and it not only took a lot of time, but was exhausting and I dreaded it. My last tests were on pieces that were scuffed using a Scotch Brite pad on a random orbit sander. It looks like it barely scuffs the surface, but it takes the shine off of an entire wing skin in no time at all with no effort. I never scuffed parts before dimpling because I wasn't sure that the primer would hold up to the dimpling. That's not the case. Scuffing before dimpling is 100x easier and no concern at all. For parts that just get covered and sit there inside and never get touched, I'll scuff, but I won't lose sleep over hitting every square inch. Those parts would be absolutely fine with no scuff at all. That's exactly what Stewart Systems said when I asked. I will make sure I scuff any parts that need the toughness factor though.
When coupled with Devilbiss Dekups, the EkoPrime becomes almost as convenient as a rattle can primer because it's waterborne and doesn't have a pot life (where it is only good for X minutes after mixing the catalyst in, like an epoxy primer does). I mixed up a batch of primer a month ago, meaning I added a tiny bit of water, and I didn't use all of it. A few ounces were poured in a small mason jar, and the rest stayed in the flexible plastic Dekups container - all of the air pushed out and the plug put in. Yesterday I wanted to prime a single part, so just pulled out the Dekups container, gave it a little swirl to mix it all up, and primed the piece. Clean up is a 60 second task with hot water. The small mason jar is what I use with a Qtip to hit random areas. It's still good to use a month later. I love the waterborne primer for this reason. The lack of the toxic smell that epoxy primers have is a big bonus as well. The epoxy primers are definitely tougher and offer better corrosion protection (they actually create a barrier over the primer that keeps water out, whereas the waterborne primers technically don't), but again, what's "best" is a very different thing than what is reasonable. Given the unprimed 50 year old planes I've seen, I figure my plane will probably last 75+ years with the "not best" primer treatment I've given it. I'm not sure I care if it lasts 150 years.
When coupled with Devilbiss Dekups, the EkoPrime becomes almost as convenient as a rattle can primer because it's waterborne and doesn't have a pot life (where it is only good for X minutes after mixing the catalyst in, like an epoxy primer does). I mixed up a batch of primer a month ago, meaning I added a tiny bit of water, and I didn't use all of it. A few ounces were poured in a small mason jar, and the rest stayed in the flexible plastic Dekups container - all of the air pushed out and the plug put in. Yesterday I wanted to prime a single part, so just pulled out the Dekups container, gave it a little swirl to mix it all up, and primed the piece. Clean up is a 60 second task with hot water. The small mason jar is what I use with a Qtip to hit random areas. It's still good to use a month later. I love the waterborne primer for this reason. The lack of the toxic smell that epoxy primers have is a big bonus as well. The epoxy primers are definitely tougher and offer better corrosion protection (they actually create a barrier over the primer that keeps water out, whereas the waterborne primers technically don't), but again, what's "best" is a very different thing than what is reasonable. Given the unprimed 50 year old planes I've seen, I figure my plane will probably last 75+ years with the "not best" primer treatment I've given it. I'm not sure I care if it lasts 150 years.
In terms of actual work today, it was pretty ho-hum. I started deburring all of the wing skins. I'm down to one more skin to go, then I'll move on to the leading edge. One thing I did figure out today is that removing the plastic sheets is far easier by rolling it onto a broom handle. Pulling the plastic off by hand is terrible! Rolling it onto a handle using a vice grip for leverage, it comes right off in a matter of seconds.
