Chapter 25 - Section 2

Filling and Sanding - Fuselage & Cowls


Though I like the ease of 'cheese grating' technique, I always have difficulty in determining 'when' is the right time. If I started sanding too soon and the epoxy/micro is not quite cured enough, it'll clog up my expensive sand paper. I eventually just wait till the whole thing cures before sanding. The West system is pretty easy to sand except the dust gets EVERYWHERE!



Here's some of the before and after pictures from my sanding effort.


My first pre-fill and sanding on a real part. I ended up with many pin holes and mouse bites. I was not impressed though with a lot of work.
I stuck some plastic strips between the nose cover and the fuselage, hoping to get a really thin seam - no luck. The plastic strips were too thin and will not stay straight, I ended up with a wavy line. I filled and sanded both nose covers.  
Filling and sanding the rest of the nose top. Starting to learn about controlling the width of a seam...I made a blue-foam sanding template for the rounded edges of the nose.
The seams are looking better, but not perfect. I used the tools (left) to carve out the seam line before the micro cures. The flat one is for straight lines while the small screw driver is for curved corners. Make sure the micro is firm / dry enough to stay put after you form the trough.
I flipped the fuselage over to complete the main gear fairings. Once completed, I started on filling and sanding the bottom side of the fuselage. There are a lot of seams on the bottom of the Cozy, so I learned to do the seam just at the right time.
I might have over killed with the micro filling (left). Generally, it was not too bad, just have a lot of excess micro to remove. The narrow Durablock works best for this application.

I did not take any picture on the sanded main gear leg until I finished sanding the fuselage bottom.

I never liked my nose gear that was shaped to fit the Feather Lite nose gear cover. I decided to add 4 BID to cover up the original gear cover and seams. 
Next, the bottom of the nose - a bit wet with the micro. I have to wait till the micro dries before sanding. It turned out OK. A bit aggressive on a couple spots. I will do some secondary patching later on.
I removed the air brake to fill and sand both sides. Then I sanded its edges as straight as possible. I taped all edges for seam preparation. Then I microed and sanded its surrounding areas.
The big day finally arrived to fill and sand the rest of the fuselage bottom. It took me almost all day to fill the bottom. I had to pay special attention (and applied more micro) along the rounded edges because I was too aggressive in sanding it down during the foam shaping stage. I sanded it down the next day. I had to follow up with additional patching and sanding to smooth out imperfections.
Sanding the fuselage bottom generated a huge dust storm. The light beam through my exhaust fan turning the work place into a 2,000 year old Egyptian Tomb - no treasure here though...but I was anticipating Indiana Jones jumping out from one of the dark corners anytime now...
Filling and sanding the NACA scoop and back end of fuselage. I left the tail-end un-filled (for now) for transitioning to the cowlings later on.
One small step at a time...the back half of the fuselage. I found a 1 1/2" plastic spreader that worked out really well for me. I was able to spread a relatively thin layer of micro onto the surface. That minimized the subsequent sanding effort.
I have been aggressive in removing micro from previous areas and ended up re-patching them multiple times. I have to remind myself to stop sanding as soon as the high points become visible. The bottom strake turned out much nicer. I also made a sanding block (with blue foam) using the R33 profile as a guide for the leading edges. I will fill and sand the spar bottom later - together with the wings attached.  
Fill and sand the right wing bottom the same as the left. I sanded the foam really smooth before glassing and peel-plied. The surface was quite smooth already, so filling and sanding was quite trouble free.
  Here's a nice sanding technique I learned from Wayne Hicks. It can be used for sharp corner treatment as well as tight round surfaces. Wayne applied it to make a nice defined edge/corner for my fuel sump area in the picture above. Simple and effective! Don't forget to go 45 degrees criss-cross pattern on the tight round surfaces.


I returned to filling and sanding after I installed the engine (Chapter 23) and its related sections. Since the cowlings are part of the fuselage, I decided to record my experience below. As most Cozy builders, I bought my cowls from Featherlite, They are brown in color and have a smooth external surface and rough (glass weaved) internal surface. My initial thought was that the cowls (upper & lower) should be pretty simple to prepare for painting, since it already has a smooth surface. - no micro and minimum sanding to shape. On the other hand, the inside...well - no one is going to see it except the engine.


I started with 80 grit. The intent was just to rough up the surfaces, remove the shiny layers and have it ready for paint in quick order. Well, not so fast Grasshopper! After I finished my first pass, while wiping the excess dust off, my hand came across some sticky gunk - feels like masking tape residue. Close examination reveals a layer of mold release - left over by the manufacturer. I later learned that they can be removed by soap and warm water. Unfortunately for me, I have partially sanded some of them off already, therefore, the mold release is blotchy (as shown on the left picture). I ended up sanding the entire surface all over again.


To make matters worse, You'll find millions of pin holes showing up on the surface. Notice the white speckles (left)? I believe these are the dreaded 'pin hole' dance as Wayne Hicks calls it! 


After hours and hours of sanding, I was rewarded with a dull surface on lower cowl. The next step is to fill those pin holes with raw epoxy and then sand again! Isn't that just wonderful?


I ended up filling the pin holes with wet micro and re-sand the entire surface. Once completed, I applied the first layer of white primer (Southern Polyurethane). Since the primer is glossy, all the imperfections were magnified! It looked terrible...

Noe helped me to sand off the first layer of primer and imperfections. Then we filled them with Dolphin Glaze. Once completed, we sprayed on the primer (the second time). It look much better, the cowls looks smooth and clean. Unfortunately, a closer look shows some more small imperfections under the light. I knew I have to clean them up - sooner or later before paint!
I decided to follow Tim Andres advise and use guide coat on the entire surface this time. I went to Lowes and bought a spray can of fast drying black paint. I slightly 'sprinkle' the paint over the entire surface. It cost less than $1 per can. I used 2 cans for the entire airplane.

Then I move to 220 grit sand paper and sand off the guide coat layer. I was VERY surprised to see all the low spots and imperfections showed up. There's just no way I can find all these imperfections by light reflections & water because they can be very small. With guide coat, you can see a bunch of black dots or patches (low spots) scattered all over the surface.

I filled all the low spots and imperfections again with Dolphin Glaze and re-sand with 320 grit. Once completed, its ready for primer - the third time! Note all the green spots on the re-sanded surface? They were the low spots that I missed - the first and second time.

The painted upper cowl on the above (left) picture looks great (after 1st prime). Now look at the picture to the right with the green glaze. That what I missed the first time without using Guide Coat!

Here's a couple picture of my fuselage bottom after the 3rd round of sanding and priming.


Here's are a few pictures on the rest of the plane after my sand, fill and prime routines.

Next Chapter - Electrical  ....