Q-talk 114 - Wing/Canard Finishing - Part 2, From bare glass to primer in a weekend
- Category: Q-Talk Articles
- Published: Wednesday, 23 December 2009 16:24
- Written by Jerry Marstall
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It was July in North Carolina when I was doing this. With the warm summer temperatures, the micro cured over night allowing me to begin sanding the surfaces the next morning.
Step 1 - Sanding. Suggested sanding tools.
1. 35" x 2.75" sanding block. Its mandatory that it be flat and non-flexing
2. 3M Supper 77 Multipurpose Adhesive Glue sandpaper to sanding blocks.
3. Round sanding block(s) Various diameters of dowels or cardboard tubes for different radius curves
4. Clark foam flexible sanding block.
5. 8.5" 2.75" sanding block Straight and strong
6. 17.5" x 2.75" sanding block - Same
Sanding supplies are expensive. Everyone says to use the best available, typically 3M. When I looked at rolls of 3M, 40 and 80-grit for $35 each, and knowing that I would use several rolls, I croaked. I understand that the better sandpaper has more uniform grit making for fewer deep scratches. Since I would be sanding off the majority of the micro, I decided to find a cheaper source for the initial sanding and save my money for the top grade 100-grit finishing paper. www.eastwood.com has rolls of 40 and 80-grit for $12.95 that works just fine for the initial leveling of the surface. Save the expensive stuff for the final sanding.
Your initial board sanding will begin with the 35" board using 40-grit. When I got the rough edges knocked off I switched to the 80-grit and the 17.5" board. You may uncover some voids in the micro surface. These voids can add a day to the process. So it behooves you to do as good a job as possible when you apply the micro. When you uncover voids, don't wait too long to fill them. If you do see some divots surfacing, stop with the 80-grit. If you wait until you have sanded down to the primer, you won't have enough micro depth surrounding the void to sand as you dress down the patch. If done correctly, you will not be able to tell where your patch was applied. Make sure you use the same ratio of epoxy and micro for the patch that you used in the application of the original layer.
One of the secrets to a good finish is using long, slow strokes. Be patient. The tendency is to use short and fast strokes. This doesn't get the desired results. Do it as Mr. X describes. The initial sanding with 40-grit took about 1.5 hrs per surface. It was a great feeling to have a smooth wing surface in such a relatively short time.
Note: For the unenlightened who are still building taildraggers, you will want to support the fuselage to get the gear off the ground. Even without the engine mounted, the weight of the fuselage is enough to bow the canard slightly. It will be difficult to produce a straight surface if the canard is bowed under weight.
The following passes with 80 and 100-grit go even faster. I used the 17.5" board with the 80-grit. That allowed me a little better control for pulling micro off areas that were thicker than I wanted. If you pull the board very slowly toward you, you can actually feel any high spots through the board. Just apply more pressure to that area on your next stroke. Slower is better.
If you will periodically brush the sandpaper face with a shop brush, it will clean the sanding dust from the grit allowing more effective cutting with each stroke. Also keep the surface free of dust so it doesn't clog up the sandpaper.
When I got to the 100-grit, I went back to the 35" board. This was to smooth out any slight ripples I might have made with the shorter board.
The flexing foam sanding board ( I glued two pieces of Clark foam board together which made it easier to hold on to and also less fragile) is perfect for doing curved areas like the wing leading edge.
The 8.5" sanding block is good for the initial feathering of patches. I again stress that the technique that insures that your patches feather in smoothly is to make the ratio of epoxy to microballoons exactly the same as the initial layer of micro you put down. It stands to reason that if your patch mixture contains more epoxy, it will cure harder and be more resistant to sanding than the softer area surrounding the patch. As a result, low areas around the patch are created as you attempt to feather the patch in.
Use the same grit sandpaper that you last used on the surface. Continue to use long 45 deg strokes in both directions. The tendency is to make short rapid strokes to smooth out the patch. If you do this, you are literally digging yourself a hole. When you get it close to level with the surrounding surface, switch to one of the longer boards to perform the final leveling of the patch.
If you were careful in your application of micro and didn't have to repair any divots, you are ready to put on your UV coat and primer. If you hop to it you can get this done before the sun goes down. What a great way to end day two. Even if you do have divots, you can repair them today and do the finish sanding tomorrow. So it took you a three-day weekend instead of two. So what. Look at what you have accomplished. Congratulations!
DAY THREE - Optional
Mr. X does mention another process to seal the surfaces. This step will certainly extend the 2-day project another day, but you may still think it is worth it. I did.
Step 1 Applying pure epoxy. The
primary reason for applying several coats of pure epoxy (no microballoons) is to fill pinholes and sanding scars. It is very effective. I saw Sam's wheel pants where he filled 36-grit scars with five coats of epoxy. My first impression was that this is adding a lot of weight. If pure epoxy was used to fill 36-grit scars on a total airplane, I am sure it would add up. I chose to sand with 100-grit and applied only three coats of epoxy. With this approach I don't feel that additional weight is a factor because, one, I am filling smaller pinholes and scars, and two, I will be sanding the top surface off with 220-grit in preparation for the final UV coat and primer.
Remember that I am working in July in North Carolina. I pulled the plane outside in the sun. Using a rubber squeegee I put on three coats in 2 hours. It took about 35 minutes for a coat to cure to the point of still being tacky but not so that I couldn't spread on a new coat. So far I haven't seen any pinholes.
Make sure that you put masking tape on the underneath side of the leading edges of the wing and canard. Attach one edge and let the other edge hang down to form a dam. If you don't, when you apply the epoxy it will run to the underside of the leading edge and leave you with large drops of cured epoxy to sand off. (Wonder how I know?)
While sanding the epoxy coat you will get a very vivid view of the quality of your sanding process. The cured epoxy is shiny and the low areas will glitter in the light. But rest at ease. If you were faithful to the above mentioned techniques, these slight imperfections will not be seen once primer is applied.
Even with all of this, when you see my plane you will no doubt find some flaws. You must understand that those were planned. If it were perfect, no one would believe that I did it.
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