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Planning
Do you really have the time to complete this project? Based on the work of others, I estimated that my fuselage-refinishing project would take about 200 hours and would be completed over a 4-month period between November and February. My base hours estimate was about right, but I performed a complete gear system cleanup, installed new tail tank and 'P' systems. And did some other things that easily bumped the time up another 50 hours or so. Although 250 hours is just 15 hours a week over four months, I actually could not spend that much time on the project and completion ran late.
You will also need a place to prep, paint and finish the ship. I did the initial gelcoat sanding, prep and painting in a garage during the late fall, and did the final sanding and polishing in my basement. The latter allowed me to work through the winter. Based on the work of others in my club, I had a good sense of the skills, materials and equipment I would need to complete the job.
You have to obtain gelcoat, solvents, spraying equipment, and sanding supplies well in advance of the project. I ordered German wet/dry from Eastern Sailplane (ES) because it has superior cutting and staying power. I also installed a 220-Volt outlet in my basement and purchased a compressor to run the air- powered sander (jitterbug) used for final sanding. (An electric powered sander is fine for gelcoat removal ... you need a non- electric (air or elbow) sander for the final wet sanding.)
Having experienced people around or accessible to guide you through the process is essential. The work you are doing could impact the structural integrity of your ship and has to be examined and monitored by people with the appropriate credentials. In addition to local A&Ps, I was fortunate to have someone around who worked at Schempp-Hirth building and repairing gliders.
Gelcoat Removal
Removing the old gel coat quickly without damaging the underlying glass structure is the first task. The experience of several refinishing projects indicated that using a random orbital sander with a very coarse grit was the most effective way to achieve these dual objectives. How coarse? Well, how about 10 - 15 grit. The 'rocks' on the paper appear to pulverize the gelcoat, but bounce off the underlying fiberglass. The orbital sander also appears to offer additional protection because the velocities and heat generated by the orbiting pattern are low compared to a rotating wheel sander.
Since all of my gelcoat cracking occurred on the top and bottom seams where the fuselage was joined, I planned to completely sand off the gelcoat in these areas, and then sand all remaining areas with 100 grit wet/dry to supply tooth for the final gelcoat application. However, you should seal every opening into the glider before starting to sand. This will greatly ease your cleanup problem later. I closed up the cockpit, wing root openings, and tail openings with sheet plastic, cardboard, and tape. I started on the upper surface just behind the cockpit. Gelcoat removal with the orbital/10 grit setup was easy to control. During the first 15 - 30 seconds or so, you can see the gelcoat surface roughen and powder off, another half minute takes you through the gelcoat and you will see filler (different color substrate) or fiberglass.
I usually worked an area roughly 6 inches long by 4 wide. I tried to remove the material without creating flat spots. Moving the sander across the fuselage centerline at 45-degree angle (like sanding wings) seemed to work fine. You are sanding a curved surface, so work carefully and stop frequently in the beginning to inspect your work with your advisor. We used a 10-power magnifier to check the glass surface, but never found any evidence of damage with this method. Skill and feel will develop quickly and you will be able to move right down the fuselage. I could sand one linear foot of fuselage in about 15-20 minutes before changing paper. We also had the exhaust fans working and wore masks to deal with the dust.
Photos 1 and 2 show the top of the tail boom and nose after the removal process. The filler that remained in the low areas was examination with a 10X scope and showed no cracking, so it was left on the glider.
Painting
We constructed a spray booth in a garage using a massive blue drop cloth purchased from JC Whitney. The drop cloth hung from floor to ceiling and enclosed a work area 35' long by 15 wide. We pressurized the space using two 20" K-Mart fans mounted in a box approx. 20" wide x 40" high x 10" deep. Furnace filters were mounted behind the fans. The box sat on the floor in an opening cut in the drop cloth. The drop cloth was screwed to the box perimeter to make an air tight seal. The fans pulled air from the adjacent garage area through the furnace filters and this pressurized the work area with air free of airborne particles. The drop cloth also had an opening for an exhaust box containing just furnace filters to trap overspray. We had no overspray in the air and very little residual overspray in the next garage bay.
Simtek Sanding Surfacer (a filler) was sprayed on the top and bottom sanded area to restore the desired contour. The top was painted and sanded first, and the fuselage was flipped to repeat the process on the bottom. The entire fuselage was then sanded with 100 grit/wet dry to provide a consistent surface for the final gelcoat application. I actually had to spend a little more time with the filler step to take care of The Dreaded Pinholes. Pinholes can have many sources related to the physical surface being painted and the fluid properties of the paint. The end result is a small void in your finish. I did a close inspection of the filled surface and use a small paintbrush to fill every void I could see. Although this seemed like an extravagant time consuming bit of work, I wound up with a final finish that only had a dozen pinholes over the entire fuselage. These were easily fixed during the final sanding phase.
Painting the entire fuselage in one session was the goal. Some factories mount the fuselage in a rig, which allows free rotation along the fuselage centerline. We used a similar concept and fabricated a cradle that gave use enough rotation so we could spray all surfaces non-stop. The 20 was suspended by the rear carry through on a frame constructed from 3/4" pipe. A small line at the tail allowed the entire fuselage to be suspended approximately 2 feet off the floor. A weight hung off either side of the rig caused the fuselage to list 45 degrees, which allowed painting of the top and bottom urfaces. In addition, we were able to move completely around and under the fuselage and not worry about tangling hoses in the supporting fixtures.
The actual painting process was pretty anticlimactic. The spray booth was set up, fans were turned on to clean the environment, the floor was wet down to trap dust, the fuselage was wiped with acetone remove remaining surface material, and the gelcoat was mixed and sprayed. (We used american-made Simtec Prestec ... a catalyzed polyester coating.) We moved continuously around the fuselage, starting with a tack coat, and tipping the fuselage from side to side, adding coats on all surface until the desired film thickness was achieved. Basically, you spray the gel coat until it is just ready to run. Runs are no problem ... just get it on thick sand off later. We wore high quality masks, overalls, gloves and goggles.
The fans with furnace filters did an excellent job keeping the suspended particles out of the air. One the spraying was complete, we cleaned up and let the fuselage hang for 24 hours to dry. The surface dried to a hard, white, shiny, orange peel. I brought the fuselage home a day later and put in my basement for final sanding.
Final Sanding
This step takes some time, but it is really straightforward. You have to sand the new gelcoat with 400, 600, 800, 1000, 1200, and 1500 Grit Wet/Dry to get the final surface finish and gloss.
The first sanding with 400 grit is hardest because it takes off the most material. Apply a disclosure agent to let you easily see the unsanded surfaces. I mixed red dope with dope thinner in a 1:3 ratio and wiped the mixture on all surfaces. Literally looks like hell ... but you can't do the job properly without it. I also fabricated some trays to place under the fuselage to catch the runoff. A 6' x 2' furring strip frame covered with plastic drop cloth makes a nice tray.
I sanded with an air-powered jitterbug, sanding at a 45-degree bias to the fuselage centerline. This is the same type of pattern as wing sanding. As you cut through the high points of the orange peel, you will see a spotted pattern developing. This of course is the bottom of the peel and you have to sand to his layer, but no further. Work in overlapping areas. The work goes fairly quickly ... I was able to perform an initial sanding from about the trailing edge of the wing root fairing to the fin in about an hour. This is actually just the top 40% of the boom ... the sides and bottom take about the same time ... perhaps a little less on the sides.
This photo shows the pattern that you first see as the top of the orange is sanded off.
The disclosure agent will also identify pinholes and areas that need some touch up spraying. Pinholes can be fixed with a dab of gelcoat on a fine brush. Clean the disclosure agent with dope thinner first ... and rough up the hole so the micro patch gets a good bite. If you hit a larger defect, you can build up the area using an airbrush with the 'big' nozzle. So, as you're sanding, be prepared to stop if 90% of the surface looks fine. You could wind up sanding all the good stuff off, which simply means you will have a bigger area to repair.
I did all the sanding for one grit first. You have to clean everything as you change grits to prevent contamination. Residual coarser grits will mar the surface of the following fine grit sanding.
I ran into some problems on the fin when I sanded through the gelcoat on the sharp leading edge at the top of the fin. Took a couple of re-sprayings with the air brush before I got the sanding technique down. The concave sections around the fin/fuselage joint and the wing fairing require the use of a hard flexible backing for the wet /dry sandpaper. A hard backing is required on all sandings to create a smooth finish. Never sand with your fingers!
The 600 - 1200 grit sandings are essentially the same. However, instead of the dope mix, I used a soft lead pencil as the disclosure agent. Simply scribble hatch marks all over the fuselage and sand off. Check you work periodically with a 5 - 10X magnifier to see if you are removing all the scratches from the prior grit.
Buffing
This is the fun part! All your hard work will be rewarded as a gleaming new fuselage comes to life in front of your eyes. Yes, all the imperfections that you know about are still there, but the buffing makes it hard to see them under fluorescent light, and impossible to see in the sun. I have used the giant factory buffing wheel and a home-brew wheel (made by stacking 10" cotton buffing wheels) with equal success. I actually prefer the smaller wheel because it is easier to handle. I used a couple of different rouges and waxes, but the hard wax supplied by ES seemed to work the best.
Buffing is actually a skill in itself. You can 'burn' the gelcoat if you allow the wheel to sit on one section too long. This is easier to do along the sharper edges like the trailing edge of the wing fairing. Just sand off the discoloration with 1500 grit and re-buff more carefully.
Buffing -Addendum
The following finishing technique was discovered two years later when I refinised my wings. I suggest this method for fuslage refinishing also.
The finishing work (on the wings) went very smoothly from this point. I used the air file to do the 400, 600 and 800 grit wet sanding. At this point, I did some experimenting with the 3M Boat Refinishing products Richard Kellerman 'discovered'.
This 3M page has an extensive list of gelcoat polishing products. As directed, I used the 3M High Gloss gelcoat Compound which "is designed to quickly remove P600 dual action or 1000 wet sanding scratches from production gel coat." This produced a reasonable shine, although I could still see 800 grit scratch marks. I then used 3M Finesse-It(TM) II Finishing Material, which was designed to removes light to medium oxidation on gelcoat for boat reconditioning. This stuff worked great bringing out the finish on Richard's new ASW-27. It also brought up the shine on my re-finish, but I could still see some scratches. I concluded that the compounds alone could not produce the finish I desired. I therefore continued wet sanding the wings through 1000 and 1200 grits before using the compounds. The results were excellent. I will never again use the large buffing wheel and hard wax utilized by most glider manufacturers.
These compounds were applied using the pad systems recommended at the 3M site. I used a big DeWalt Sander/Buffer (DEWALT DW849 7/9" 0-1000/3000 RPM Variable-Speed Electronic Right Angle Polisher) to drive the pads at the correct RPM.The final buffing will produce a mirror like surface, but this is actually just the gloss from the gelcoat surface. The compounds actually are wax free and you have to apply a type and brand of your choosing. I used Mother's canauba, although it probably had some silicone content. And I hate to say it, but the wax will actually lower the gloss a little! The buffing and waxing took about two days.
Rudder/Gear Doors/Control Hookup Hatch Door
I sanded and painted my rudder, gear doors, and control hookup hatch door after I finished the fuselage. I actually painted the entire rudder and doors with an air brush using a wide nozzle and unthinned gelcoat. I painted the rudder flat ... one side each evening. Remember to perform a rudder balance check per your manual to insure that the overall weight and rudder mass balance is within limits.
The control hookup hatch door fit can cause you some problems if excess gelcoat has filled the corners of the recessed door opening. Patiently chisel and/or sand the excess until the door fits properly.
Final Assembly
Mount the canopy and check the fit. I sprayed my canopy rails on the fuselage and had to do some sanding to get the fit correct. Put all the pieces together and check 3 times. Apply seals, N numbers, contest numbers, manufacturer decals, do a weight and balance and have the ship inspected.
The only thing you have to do after this is go fly! ... and answer questions about how you spent your winter vacation!
I want to thank Richard Kellerman, Bruce Conrad, Eastern Sailplane, and M&H Soaring for providing the information and help to make this project possible.
Simtek
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Bldgs. K&L
Anaheim, CA
92806
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