FIBERGLASS FINISHING TECHNIQUES

by Lyman Slack

 

        I recently faced the project of finishing my Bonanza model. When I last used a fiberglass finish, it was many years ago and involved polyester resin, which was smelly, messy, and weighed a ton (not to mention that it was horrible to sand). However, master craftsman Ray Helpling was glassing his huge DC-3 at the same time. Ray was kind enough to allow me to go to school on him and learn his technique using 6/10oz or 3/4oz cloth and Pacer Z-poxy.

 

          First off, prep your wood surfaces as smooth and ding free as possible; I go down as far as 320 grit paper. Fill any dings and hangar rash with Balsa Magic Filler.

 

        Have your supplies on hand: Z-poxy Finishing Resin; old credit cards or similar, small mixing cups (one ounce medicine cups), stir sticks, acetone for clean up, a stack of paper towels, good sandpaper, etc.

 

        Start by using a compressor to blow off all the dust, etc. If not available, use the brush that comes with a vacuum cleaner to vacuum off the surfaces. Lastly, use a tack cloth.    

 

        Start on the lower surfaces; let's just use the wing in this example. Cut a piece of your cloth so that it is about two inches larger than the wing outline. Lay the cloth on one half of the wing, overlapping the center line.

 

       Here’s a hint courtesy of Dave Platt: If you have an old draftsman's brush, gently brush out the glass cloth; the static electricity really makes the cloth lay down nicely.

 

        Mix you Resin. Don't be too concerned with getting an exact 50/50 mix; it isn't rocket science. "Close enough works!” Mix only a half an ounce at a time; that's probably enough for one half of a wing of a model with a 6' span. Don't worry if you run out; you can always mix more as you go.

 

        The fun part: Start "drizzling" on the resin by pouring a couple of lines of it from tip to root either side of the center chord. Get your squeegee card and start spreading it starting in the center and working out to the LE & TE; keep working it and you will be most surprised at just how much coverage you can get.

 

        When you get to the LE & TE, just let the cloth drape down; squeegee slightly around the rounded LE. You can also use an acid brush for this step.

 

        No go back and squeegee everything again; you'll find you will be getting more and more resin building up in front of your card, like scraping a knife across a stick of butter. Use it to blend into dry spots or wipe the excess off on a paper towel. When you are done, the surface should look pretty dry, smooth, and with not too many wet, shiny spots.

 

        Set the wing aside and allow curing overnight. The next day, sand the end of the cloth near the center line with 100 grit sandpaper, clean as before, and do the other half.

 

        The next day, take your sanding block and while holding it at a 45 degree angle to the wing surface, sand down the LE & TE and you will see the excess cloth just drop off clean as a whistle. Sand down any excess resin alone the edges and repeat the process on the top of the wing.

 

        When it's all done, it's sanding time. I have found that 3M brand Wet or Dry works best; you can get it at any good Auto Paint & Body Supply shop. It's the black stuff. You will use it dry.

 

        Use only a block, not freehand, and start with 100 grit. It will sound horrible as you start, but gets quieter as you cut the resin surface. The goal here is to get all the resin on top of the cloth removed without cutting the cloth. If you cut into the cloth, don't fret; it'll be covered with the next coat of resin. After the 100 grit, move to 180, and then down to 320 to really smooth things out. Here, and at this stage only, I allow myself to use  a sanding pad -- actually what's called a "Softback Sanding Sponge" made by 3M; part # 02604 - Fine. These are really neat 1/4" thick foam sponges with an abrasive on one side. Your glassed surfaces should look dull-smooth, and feel as smooth as a baby's tush at this stage. Clean all the surfaces.

 

          No one is perfect – chances are that you will find a few high spots and sand right through the cloth. Not to worry! If you have only sanded into the cloth, the second coat of resin will fill the weave. If you have actually sanded through the cloth and into the balsa, here’s another one of Ray’s tips: Drop a couple of drops of thin CA into the bare wood spot; rub it around with your finger, and while still damp, sand it smooth with some fine sandpaper. Not only will this seal the wood, you will sand the fuzzies off while making the wood itself strong.

 

        Just for reference, my 81" span Bonanza wing (pretty big at 1040 sq. in. and thick) picked up 2.3 ounces, with the cloth accounting for one ounce.

 

        Stage two: pour on a second coat of resin and squeegee MOST of it off; all you are doing here is filling the weave and any remaining low spots. This coat gets sanded by starting with 180 grit and ends up with your sponge. Very carefully look for any spots you might have sanded through. If you find any, reapply some resin and sand out.

 

        What you have done at this stage is to give yourself a smooth-as-glass surface with as little weight added as possible. The second coat of resin added only 3/4 oz to my Bonanza when sanded.

 

          The real good news is that you only will need one coat of that heavy primer now, and it'll be almost 90% sanded off. ALL PRIMERS ARE HEAVY by nature; the resin/cloth has replaced many coats of primer.

 

        I always use the old K & B SuperPoxy primer, which is no longer available, but any brand of primer compatible with your color coats will be fine.

 

        I might suggest you try a simulated airfoiled and curved outline "wing" first just to get the hang of it. Let me know how the technique works for you, and any changes you might suggest will be appreciated.