Sport Aviation 91

COMPOSITE PARTS REQUIRE

careful preparationbefore painting. On mostcompleted compositeparts the supporting fab-ric’s weave is still visible,and filling it is the firststep to the smooth finishcomposite airplanes areknown for. Intuitively,you probably know that their mir-ror-smooth surfaces are the result ofa lot of hours of labor. Many experi-enced builders say that a third oftheir total building is dedicated tofinishing.

Most homebuilts that use differ-ent construction methods, such aswelded steel tube, wood, or sheetmetal, have some composite partslike wingtips or cowlings, and thefinishing process described hereworks for them as well as it does fora composite airplane. The only dif-ference is size.

Properly preparing compositesfor paint not only makes themattractive, it protects them from thesun’s ultraviolet (UV) rays, whichcan deteriorate epoxy resins. Oneresin manufacturer cautions that itshighest grade epoxy can totallybreak down in 15 months if notprotected from the sun. This is trueof all epoxies. The symptoms arechalkiness, followed by delamina-tion.

A sunlight-blocking primer is thebest way to protect epoxy. Whenpaint manufacturers claim thattheir products have 100 percent UVprotection, they are referring to thepaint or primer, not the substrate it

covers. Using a primer that totallyblocks the sunlight from the surfaceof the structure is a simple insur-ance policy.

Prep WorkPrepping composite parts for paintis a three-step process: 1) rough fill-ing of the weave and contouring; 2)applying a filler/primer to obtainthe final shape and fill the pinholes;and 3) applying the topcoat.

Generally, you mayencounter three types ofcomposite parts, andeach requires a differentpreparation method.

Hand lay-up or vacu-um-bagged parts usuallyrequire a lot of filling,particularly if they’re notvacuum bagged. Typical

examples are on plans-built com-posite aircraft like the Cozy, Long-EZ, etc., or parts that are not manu-factured in a mold. Often filling andcontouring will be required toachieve the desired shape.

Molded pre-preg parts are gener-ally well shaped out of the mold,but they usually have pinholes andrequire filler primers. Little shapingor contouring will be needed. Manycomposite kit aircraft fall into this

Nuts & BoltsNuts & Bolts

Aircraft Building

The Joy of Sanding

Preparing composite surfaces for paint

RON ALEXANDER

Even a “metal” airplane, like thisRV-6, has a lot of fiberglass partsthat need preparation before paint-ing.

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92 FEBRUARY 2003

Aircraft Building

category, are as molded parts likecowlings. These pieces sometimesrequire a filler, but usually afiller/primer will fill the pinholesand level the surface.

Gel-coated parts are smooth andvirtually pinhole free because theyget their Gel coat in the mold,before the part is laid in it. A gelcoat includes a color pigment, usu-ally white, and typically you cansand the part smooth and apply thefinal primer with no fillingrequired. Some polyester gel coatsshrink over time, and some builderschoose to grind at least a portion ofthe coating off and then prime overthe sanded surface with a more sta-ble filler/primer.

Finishing StepsStep 1: Filling. A homemade mix-ture of microballoons and resincalled “micro” or “slurry” is theclassic way to fill rough areas or thefabric weave. Microballoons makethe mixture lighter, and when youadd them to epoxy it has the con-sistency of peanut butter (smooth,not chunky), then trowel orsqueegee the mixture on the areayou want to fill.

Another filler is Poly-Fiber’sSuperFil, a commercially formulat-ed, pre-mixed epoxy-based fillerthat eliminates the guesswork inmixing your own microballoons

into resin. Some builders have usedBondo in place of micro, but Bondowill shrink. And it’s heavier. Don’tuse Bondo on an airplane, unlessyou want to repaint it after a fewyears.

Before applying any filler, ensurethat the surface is clean. If it’s amolded part, make sure it’s free ofany wax or mold release agents.

Sand the surface with 180- to 220-grit abrasive paper as a final stepbefore applying the filler.

Spread the filler on the surface,but don’t overdo it. The objective isto fill any low spots in the fabricweave without totally covering thehigh spots. Don’t use any filler as abuild-up to give shape to leadingedges or fillets. Fillers are not struc-

tural materials. Any build-up offiller more than 1/4 inch is toomuch, and too much filler can crackover the years. Start with thin coatsof filler forced hard into the surface,and warm temperatures (75°F orhigher) give the best results. Asqueegee is a good tool for applyingthe filler.

After applying the filler you areready to sand, the compositebuilder’s favorite activity. A fewrules:

� Use only high-quality abrasivepapers. Cheap abrasive paper canleave scratches.

� Hand sanding is usually moreeffective than using a machinesander. It’s very easy to damage asurface by sanding through theunderlying reinforcement material.

� Spend very little time withrough grit abrasive paper. Use 40- to80-grit sparingly, and only forrough shaping.

� Gradually change to a finerand finer grit—about 100 grit at atime.

� Use straight edges to find highspots and depressions of flat sur-faces. On curved areas you can use atemplate.

� Stop sanding with about 240-to 300-grit abrasive paper. You don’twant it to be too smooth or the sur-

Even if you’re build-

ing a metal, tube

and fabric, or wood-

en airplane, you’ll

probably be faced

with finishing some

fiberglass parts.

Poly-Fiber’s SuperFil does an effi-cient job of filling low spots onfiberglass surfaces. Right, rollingon UV Smooth Prime.

091-094 Aircraft Bldg 302 1/13/03 10:17 AM Page 2

face will not hold the primer coat.Step 2: Priming. Applying a

filler/primer usually completes thefilling work by filling any pinholesin the part or the filler material, andby providing a smooth level surfaceunder the topcoat. The adage thatpreparation is 90 percent of paint-ing is applicable here. The harderyou work at getting a perfectlysmooth, level surface, the better thepaint job will look. Proper prepara-tion at this step will require fillingand sanding followed by more fill-ing and sanding until you achievethe desired surface.

There are lots of filler/primersavailable—most developed for theautomotive market. When pickingone, here are some rules of thumb:

� Use filler/primers that come intwo parts: primer and catalyst. Theyare more expensive than one-partlacquers or polyesters, but they have

a longer service life with fewer prob-lems. Two-part catalyzed primersmay be either epoxy or poly-urethane.

� Use the filler/primer recom-mended by the manufacturer of thetopcoat you plan to use. Choose thepaint you like, and then use the rec-ommended primer. Don’t invent ormix products.

� If you use polyurethane, makesure you protect yourself from thetoxic spray mist. Polyurethanerequires fresh-air supplied respira-tors. Charcoal masks are not suffi-cient. (See EAA Sport Aviation,December 2002, for additionalpainting safety information.)

There are some nonhazardoussystems: Poly-Fiber makes a non-hazardous waterborne urethane, afiller/primer, UV Smooth Prime,that you can spray or apply with afine closed-cell foam paint roller,

and a simple respirator will protectyour lungs from particles. It incor-porates a UV blocker and fills pin-holes rather than bridging them. (Ifpinholes appear after sanding, itmeans your filler is bridging, notfilling, the holes.) A primer coat willbe required over UV Smooth Prime,so spray on the primer normallyused with the topcoat paint you’veselected.

Then, it’s time for more sanding.Most primer/filler sanding startswith 180 grit and progresses to 280or 320. After each sanding carefullyexamine the surface to see that thepinholes are being filled and thatthe surface is flat and level.

Filling and priming is a laboriousprocess, and rushing through it usu-ally results in a poor finish. Onceyou start, be patient; it will taketime and effort. Make sure youapply the primers in the recom-

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Sport Aviation 93

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mended temperature range. Allowample drying time before you sand.

How do you know when you arethrough prepping and ready topaint? When the surface is level,smooth, and free of sandingscratches and can stand inspectionin direct sunlight or under strongillumination. One way to ensureyou have a smooth surface is tospray on a very light coat of blackacrylic paint. Then sand the surfaceto expose low spots where the blackpaint is left on the surface.

Step 3: Final topcoat. Whenpainting composites, in most casesyou should use a two-partpolyurethane paint. One-partenamels or acrylics are brittle anddon’t have service lives equal topolyurethane. Remember that yourtopcoat and primer must be com-patible, and always use a fresh airrespirator.

Today’s “standard”for composite aircraftis a flawless high-gloss finish. This avi-ation-society expecta-tion leads manybuilders to contractthe final painting toprofessional shops ratherthan doing it themselves. But ama-teur craftsmen can achieve the fin-ish they desire if they are willing toput in the necessary time and effort.

Polyurethane paints are availablein two primary types: single-stageand basecoat/clear coat. Single-stagepolyurethanes are one-step paints:mix and spray. Basecoat/clear coatsare two-step processes: mix andspray the flat-looking basecoat, andfollow it with the high-gloss clearcoat. Timing is usually criticalbetween the application of thebasecoat and clear coat, and the

clear coats can be diffi-cult to spray. For thesereasons, most amateurpainters use single-stage polyurethaneslike Imron orAerothane.

The real trick toachieving a flawless

polyurethane finish is not so muchtalent with the spray gun, but timespent in final color sanding andbuffing. Color sanding is theprocess of wet sanding dried top-coat paint with finer and finer gritsof sandpaper to achieve anabsolutely flat surface. Most of thissanding is done by hand and usesfinishing abrasives with grits from1000 to 2000. After the color sand-ing is finished, variable speed auto-motive buffers with polishing com-pounds are used to restore the shinefor a perfect finish.

94 FEBRUARY 2003

Aircraft Building

For more information, visit SPORT AVIATION on the Web at www.eaa.org

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