Originally Posted by tgmillso
An extract from the manual is as follows:
"The finishing procedures just described will constitute a sizable portion of the total building time. However, they are important for structural reasons as well as cosmetic. Most of these holes, edges, etc. will be inside the airframe and out of sight when the airplane is finished. This is no reason to consider them unimportant. The need for good edge finishing is most difficult to impress on new builders unaccustomed to aircraft standards.
Scratches in the surface of aluminum can have the same weakening effects as rough edges, corners and holes. The alclad sheet used is very easily scratched because of the thin surface layer of soft aluminum. Scratches within this layer will have little effect on strength, but deeper scratches will. The greatest difficulty is deciding how deep a scratch can be before it is a potential problem. The best approach is taking extra care to prevent scratches in the first place. When a scratch does occur sand or buff it out no matter how small. Very light scratches can be removed with #600 wet sandpaper. Deeper ones will require #400 (or perhaps more coarse) sandpaper, followed by #600 for finishing. One thing to remember when removing scratches is that in doing so the corrosion resistant alclad surface of the aluminum is also removed. Therefore any area that has been sanded for scratch removal must be primed"
That's a bit crazy, IMHO.
I've worked on many many aluminum aircraft over the years, of all types. I've de-riveted, fabricated new parts and re-riveted them back together along with existing products. I've seen the insides of all brands of aluminum aircraft, including those with and without previously-applied primer. I've NEVER seen aluminum components buffed/sanded/polished to that degree. Not once that I can remember.
My empennage kit has not yet arrived, so I haven't read that section of the instructions for myself. So what's the rationale for that level of smoothing (polishing) before assembly? I understand the whole "stress riser" deal, but I think that's extreme overkill...based upon several years' experience working as an A&P. Maybe someone smarter than I can enlighten me though.
As to ALCLAD, I seem to remember we were taught in A&P school that it was a layer of 1100 aluminum several thousandths thick. I've primed a bunch of parts in my life (lost count in the 1980s), and have never done anything other than some light action with medium (red) Scotchbrite. There are very light scratches visible on the surface but I've never gotten anywhere close to removing the ALCLAD layer--at least that I was able to tell. In the 80s' & 90s we would always just use a Zinc Chromate sprayed primer (can or gun), and it would suffice just fine. I owned a 1967 Piper Aztec and re-primed the inside of the thing in 2001, some 33-34 years after the thing was primed to start with. The only reason it needed it was because I had to use lacquer thinner to clean the inner skin surfaces, and that took the primer off quite readily.
So call me crazy, but I am not sure why such extreme polishing needs to be done to avoid "stress risers." I can assure you that the 50-60+ year-old Piper and Cessna aircraft that we see flying every day weren't treated to that extent. I've worked on many of them, and didn't find one whose parts had been polished with 600-grit sandpaper.
As for priming, I will be doing it as much as to facilitate easier inspection of internal structures as anything. I mean, sure...the corrosion resistance is great. But the real reason I like it is because it makes it a virtual pleasure to inspect interior structures when they've been primed: It's easier to see abnormalities when you don't have a bright LED light reflecting in all directions off a million different smooth aluminum surfaces. Been there, done that.
Interesting discussion though, as I'm looking for a primer product to use here in a few weeks.
EDIT: Re-reading the verbiage that Tom quoted from the manual, I guess I can maybe see why there are saying this. It's certainly massive overkill, but they of course have to give some guidance to "amateur" aircraft builders. So call it "CYA," or call it "providing explicit guidance to inexperienced builders," and I guess it's all the same. And I do agree with their recommendation about "deep scratches" (I'd call them gouges) weakening the structural integrity of the aluminum skin. But I do see some folks taking things to extreme on YouTube, when they are documenting the preparation they put into parts prior to riveting things together--and that's fine, as long as they're happy with the results and can feel that they've done their best during the assembly process.