Original builder Entries....
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These steps were completed on various days. I owe a lot to John here. He's not only quite good at working with micro, he's also been doing most of (well, nearly all of) the sanding, which is helping me out a lot. Sanding is the hardest part of this build for me because I've got a form of arthritis in my wrists and hands. Hopefully I can find some way to make it up to him.
2011-09-11 (1.50 hours):
Sanding...
I think I'm going to have a lot of posts with this title. I did a bit of sanding just to round out the day. The top of the wing had micro on it (used up from a previous job - who knows what, it was 2+ years ago) and I spent a bit over an hour fairing it out. I took it down in the recommended angled pattern until glass was just peeking through in areas. More later. [This ends Chad Robinson's write up on sanding and finishing. PK from here forward.]
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Finishing is one of the most time consuming parts of composite construction. Chad got the project's big pieces all done, the work of profiling the airplane surfaces. He had the engine hung, and then he sold the project to Craig. Craig pretty quickly figured out that his back wasn't going to be happy finishing off the sanding work - lots of repetitive motions and dust. I took over in 2018, and it's been 80% of the work of my first two years with this project. The software for the builder's log doesn't allow for time entries over 100 hours, so I'll have some repeat entries to get an accurate number.
The “technique” here is joking referred to as “dump and run”, followed by the “cory bird” pre-paint pinhole chase.
- The dump and run is a way of finding and filling low spots. Spray or dust a guide coat in a strongly contrasting color across a large surface (e.g., top of wing). A layer of micro (pure epoxy + glass balloons until the consistency of peanut butter) is then applied, then long board sanding until one or more spots are down to the guide coat. Don't cut the surface of the glass!
- The Cory Bird technique is about sealing the surface and avoiding pinholes in the final coat. Typically the first layers of primer are plagued with pinholes, caused by dust contamination from the underlying micro and/or fiberglass. Cory Bird is a Scaled Composites engineer whose plane “Symmetry” was celebrated as having an extraordinary finish without using heavy multiple clear coats. The Cory Bird solution is to give the (final sanded) surface 2-3 very thin layers of pure epoxy, applying and then promptly wiping off the product.
Notes on sanding tools. Below is a picture of the collection as it exists today, with notes from left to right.
1. Flooring blocks. I found some flawed 8' boards at Habitat Restore, and cut out shorter lengths. A local cabinet shop used a planer and table saw and I received good flat boards, used in items to the right. The species is Brazilian Cherry; fads in flooring lead to super hard woods coming into the US.
2. Sandpaper on PVC pipe and on closet dowel. I used these for fairings. My thanks to JD Newman for sharing his insights about the radius to be used – I was making them too small.
3. Aluminum extrusion. In Florida, pool cages are both strong (40+ mile winds, no problem) and inexpensive. Part of the reason is low cost aluminum extrusion like these two pieces, in a 2' and 4' length. They are super flat and I use them for 240 grit paper. I was startled at the big difference they made – it's a big step up. Don't live in Florida? Buy an 8' length from Home Depot, delivered free to your local store. Thanks to Buly Aliev for pointing me to these.
4. My Brazilian Cherry sanding boards, in grits of 240-120-80-36, with short and long lengths. My personal preference is for door knob handles, secured with epoxy+micro or epoxy+flox. I get door knobs at 2/$1, again at Habitat's Restore.
5. HFT sanding pad. At 17” long and a rubber pad, this has proven to be more useful than I ever expected. The pad has just enough flex for slight curves, and the super light weight helps. I have this set up at 120 grit, and there is another I will set at 240 grit for all the curves of the fuselage.
6. HFT rasp. The group archive told me about using the rasp on green stage micro, and that worked well. But the micro is abrasive and destroys these things – this is my 3rd one. HFT does not sell replacement rasp screens, so brace yourself for having to throw the whole thing away (or if you feel you did not get your money's worth, HFT _will_ let you exchange it for a new one).
7. HFT roll sandpaper. Yes, it clogs fast, but that's reason for me to take a break and brush out the clogs. Both sandpaper and brushes are super cheap – see
https://www.harborfreight.com/2-34-in-x-25-ft-120-grit-sandpaper-roll-63333.html and
https://www.harborfreight.com/6-Pc-Detail-Brush-Set-69526.html 8. Loose sandpaper. We all have it – my objective is to burn through the collection from 20+ years of buying too much, or buying an assortment when all I wanted was one grade.
9. Respirator. I had one I didn't like, then this one – 3M's M7502 with their P100 filter. You buy the mask and filter separately, and I paid $27 from Amazon.
10. Not shown are my elbow-savers. Between past sports and work, it's smart for me to wear an elbow brace – they are $5 each on ebay.
11. There is some automation under the table – a quarter sheet sander and an 18” belt sander. The first part of the epoxy wipe method is to put on a thick coating of micro. A light pass with the electric tools saves 30 minutes of hand sanding. That said, the 36 grit is almost every bit as fast – I have done it both ways and ended up preferring 36 grit board. Those tools stay under the table, unused.
What I've done wrong…
• Moving up in grade too soon – it leads to too much time with too high a grade sanding paper. The efficient path is to get close, then move up in grades to remove the course grade sanding scratches.
• When using sandpaper-wrapped tubes, focusing on the center of the filet rather than edges. Trying to create a nice smooth curve, I ended up cutting into the flat I wanted to preserve.
• Trying to do epoxy wipe with a plastic tool. The process worked much better after I bought the rubber squeegee (
https://www.aircraftspruce.com/catalog/cmpages/comptoolrubbersqueegee.php?clickkey=254482) .
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Chad's work was half of dump and run with no epoxy wipe layers. Craig left no details of his finishing work. I found that there were significant high and low spots over the airplane, so I repeated the dump and run and followed it with Cory Bird.
The work was done on a disassembled airplane, and I got faster and better as I practiced. Time estimates are as follows:
Gear Leg 10 hours
Canard 15 hours
Right wing & rudder 35 hours
Left Wing & Rudder 30 hours
Fuselage bottom 20 hours
Fuselage top 40 hours
Control surfaces 20 hours
Canopy in & out 10 hours
It is the !@#$! nature of this work that it has lots of small tasks with large blocks of down time in between. If I had the 4 arms of Vishnu I'd be able to work multiple pieces at one time. As it is, there is a slow and steady process of applying product and sanding it off, perfected on early parts then repeated. Don't sand the next part while the epoxy is hardening on the prior one - you end up with sanding dust in the curing epoxy. Don't work the epoxy past it's runny liquid stage. Don't use a fast set pure epoxy on a hot day. Wear the mask! These all sound obvious, but I tried to bend each rule and regretted it.