Skin Rivet Hole Prep

Discuss painting, polishing, or painting vs polishing.

Skin Rivet Hole Prep

Postby pfhoeycfi » Fri Mar 30, 2018 8:32 am

Curious as to how other builders prep the holes before riveting the skins. Are you just deburring? Or are you also using scotch brite on the holes as well.

Are you prepping the remainder of the skins in any way in preparation for painting, and or polishing?

I guess I'm looking for a tried & true procedure to follow. I am leaning toward polished finish...but haven't decided for sure...may be a comb of paint & polish.

Thank you,

peter h
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Re: Skin Rivet Hole Prep

Postby Rynoth » Fri Mar 30, 2018 8:46 am

I used a deburring tool (feel every hole with your finger to be sure it's clean), and then a pass with a fine (grey) scotch brite hand pad.

On the skins, I used scotch brite hand pads around their perimeter, and used a file to put just tiny bit of roundness in all the corners. I suppose the philosophy is just to avoid any true 90 degree corners, including those on the edges of thin material. Everything gets a little rounded.

For polishing prep, I just made sure any scratches were reduced to nothing more than what a grey (fine) scotch brite hand pad would leave.
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Re: Skin Rivet Hole Prep

Postby pfhoeycfi » Fri Mar 30, 2018 10:14 am

Rynoth wrote:I used a deburring tool (feel every hole with your finger to be sure it's clean), and then a pass with a fine (grey) scotch brite hand pad.


ok great. I have the maroon pads, I'll order some grey pads.


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Re: Skin Rivet Hole Prep

Postby Sonex1243 » Fri Mar 30, 2018 12:15 pm

I used a deburring tool as well, then hit the holes with a maroon scotch brite hand pad. I also hit the edges of the skins lightly with a 320 grit sanding block and scotch brite to remove the laser cut edge and barely round the edges. I also use (due to my laziness) a dremmel motor with a 220 grit flapper wheel to deburr everything with, just hit the hole real light and follow up with the scotch brite. Just don't use the dremmel on the side you are polishing! Afterwards, I wipe everything down with mineral spirits and a micro fiber cloth prior to riveting.

If you are going to polish, I would suggest doing the skin polishing while flat on the table before riveting, much easier.
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Re: Skin Rivet Hole Prep

Postby NWade » Fri Mar 30, 2018 12:51 pm

After 1300 hours of building, I've found the Maroon pads to be plenty good enough.

The basic test is if you drag a fingernail across any mark or scratch in the material (at 90 degrees to the scratch), can you feel the scratch with your nail? Does your fingernail catch on the scratch, or jump across it? If so, the scratch is too deep/sharp-edged and needs to be polished out (if possible).

In my experience, Maroon pads don't leave scratches deep enough to be a problem and they require fewer passes than a gray pad when you're trying to buff out scratches and/or leave a consistent "satin" finish for later priming (if you go that route).

As for deburring: I just used a speed-deburring tool and made sure I could my fingertip (the fleshy part, not the nail) across the holes and not feel any sharp edges or burs. Its easy to over-do it with the deburring tool and countersink thin material, so you have to be careful. NOTE: Aluminum is soft enough that you can do this test with your finger. But do *not* try it with the steel or titanium components - those metals are hard enough that you'll slice your hand or get painful metal slivers in your finger (ask me how I know!)

Clamping materials tightly before drilling and using some wood (blocks or the surface of your workbench) to back up the drilling will do a lot to cut down on the burs!

If you decide to prime/paint the aluminum, here are my steps - which have worked well and resulted in good adhesion:
  1. Remove protective plastic from all areas of your part that you need to mark for cutting/drilling (the plastic moves on the metal, so marking the plastic and cutting or drilling through it will result in improper dimensions/locations).
  2. Do all necessary cutting, bending, drilling, and deburring.
  3. Test fit and cleco parts together until you've built up an entire assembly (a whole flap, a fuselage side, etc).
  4. Disassemble, remove any remaining protective plastic.
  5. Use a hand-file, scotchbrite belt, or scotchbrite wheel to round all corners and smooth all part edges. Note that despite their high quality, many pre-cut parts from the factory have some "chatter" on the edges that you need to file down to avoid having stress risers. This is normal and documented in their instructions.
  6. Use a scotchbrite pad (I prefer Maroon) to scuff all sides of each part. Use a circular motion and/or move the pad back and forth in different directions. You are aiming for a "satin" finish that looks fairly uniform and not dominated by "brush strokes" in just one direction. However, note that the scotchbrite pad will visibly mark up a part more when going across the grain of the metal than when you move the pad along the grain. So if you use the pad on a part and are not seeing any progress, try going at 90-degrees and you'll get a lot more visual feedback.
  7. Now you'll have a part that's able to take paint/primer, EXCEPT it has lots of metal dust from the Scotchbrite pad, and lots of oil and dirt from your hands/gloves (and the hands of every factory worker who handled the raw material before you). So you need to clean that off...
  8. My preferred method for cleaning parts starts with a warm-water bath with a little Dawn liquid dish-soap. I do this in a big utility sink in the garage/laundry-room, or in a big tub of water outside on the driveway. When the parts are really big, I set the parts up on saw-horses and use the hose (although the cold water is less-effective and quite "refreshing" when the weather is cold).
    • I wet the parts and dribble a little soap on them.
    • Then I give them a good "massage" with my hands. I find that wearing latex gloves makes the process a bit more-effective; but messier as water inevitably gets in the gloves and some parts are still sharp-enough to cut the gloves (even if they don't cut my skin).
    • I massage each part to work the soap into every side/surface and get it lathered - I want the soap/bubbles to help lift the oil and dirt from all of those little scotchbrite-induced scratches.
    • Next comes a warm-water rinse of the part, again massaging it as much as possible to ensure that all of the soap (and its trapped grit) are lifted and rinsed off.
  9. The parts are dried with clean towels (preferably disposable/shop towels - wasteful but ensures I'm not simply transferring grit or oil from one part to the next).
  10. I return the parts to the workshop and rub them down with disposable shop towels (or clean rags) that have some zero-residue cleaner on them (like economy thinner or denatured alcohol or similar). If the towel looks dirty after rubbing down a part, I'll repeat this step a couple of times to really get the part as clean as possible - every bit of trapped grit means less paint adhesion. When possible, I only handle the parts with latex gloves after this stage, to avoid adding skin oils or dirt back onto them.
  11. I let the cleaner evaporate from the parts for at least 5-10 minutes before painting them.
  12. If you use a spray-on primer, it is VERY easy to overdo it. Don't try to get an even coat of paint in just one go or you'll wind up with heavy paint application, runs, and poor adhesion (ask me how I know!!). Spray lightly across the parts and don't stress if you miss some spots or get uneven coverage. Let them dry for at least 5 minutes, then go back and spray another light application. If you missed some spots or had uneven coverage the first time, try spraying from a slightly different angle on the second pass. Repeat as-necessary, flipping the part after its dry to the touch (somewhere between 15 minutes and a couple of hours, depending on the paint system you're using). You want to wind up with a light film coating over all sides of the part. You're aiming for more coverage than a spattered appearance, but much less than the thick "coat" you'd want with latex paint on a fence or house exterior. Lastly, note that spray-on primer will be dry to the touch quickly, but will not fully harden or adhere to the metal for several hours and becomes more scratch-resistant as it cures (even extending over a couple of days). If you try to assemble the parts as soon as they are dry to the touch you may find that simply assembling the parts with clecos is enough to scuff primer off of the metal. You can do it, you just have to be careful! Be patient, if you can. I like to finish my night's work with the priming so that it dries while I sleep

I'm probably a bit more anal than some when it comes to my cleaning process; but I've been really happy with the results. I hope this rundown helps, and good luck with your project!

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Re: Skin Rivet Hole Prep

Postby Bryan Cotton » Fri Mar 30, 2018 2:55 pm

My Waiex is polished. I used a deburring tool, then polished, then dimpled (when applicable). Maroon Scotch Brite is great if you are going to the dark side (paint). No sense in scotch briting under a rivet head unless you are painting or priming before riveting.

On non-polished parts inside, I just deburred. If a couple turns off the deburring tool were not sufficient then a couple swipes with a file or the 90 degree air sander was used to clean up the burrs.
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Re: Skin Rivet Hole Prep

Postby inventor » Sat Mar 31, 2018 10:21 am

This is great information. I haven't done any painting yet but plan to use an Alodine dip for my small parts. It is difficult to do on large sheets. I did the Alodine on my main spar components but the holes cause drippage through to the other side and I had to be careful to rinse it off before it sat on there too long and caused streaks. I don't want to use the Alodine coating on the outside of the aircraft because of the necessity of acid prepping before the Alodine is applied and the seepage through the rivets is worrisome. I plan to primer the inside of my aluminum sheets before assembly and the Scotchbrite method sounds like a winner.
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Re: Skin Rivet Hole Prep

Postby kmacht » Sat Mar 31, 2018 11:28 am

I painted. For prep I hit the outside of the skins with a maroon scotchbrite pad. Putting it in a palm sander made short work of it. After using the maroon pad I wiped down with acetone and assembled. Before painting I used a water based product called alumiprep. You spray it on and wash it off with a hose. Right after that it was primed and painted.

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Re: Skin Rivet Hole Prep

Postby DCASonex » Sun Apr 01, 2018 11:10 am

Since you mentioned possibly polishing, I strongly recommend doing that on the flat sheets that can be laid out on a table after all holes and trimming is completed. Reason is that you can bear down on the polisher without fear of leaving imprints of internal structure on surface, and being able to apply some pressure greatly seeps up the polishing process.

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