Sanding down the balsa plane body before putting on tissue paper.

Sanding and I have a love/hate relationship. I hate sanding, but I love the results. There's nothing better than finishing a well-sanded piece of furniture. But we digress, this is about covering well-sanded balsa plane model bodies. You have to sand the body because it is unlikely that the stringers and the formers will be perfectly flush after assembly. If you do get the formers and stringers perfectly flush throughout the entire body, then I want to know how you do it!

My favorite way to sand a balsa plane model is to wrap a piece of 100 grit sand paper around my pointer finger. I use the roll sand paper that has adhesive on the back. It is actualy made for a Porter-Cable random orbit sander, but is perfect for this appplication.

A piece of sandpaper stuck to my finger is perfect because I get tactile (ie, touch) feedback on whether or not the pieces are flush. Basically, I put the sandpaper on my finger and then move my finger back and forth until the pieces are flush. The back and forth motion is alot like waving a finger in a "no no no" fashion at a toddler. You'll feel that the pieces are flush when the "no no no" motion is smooth from start to finish.

It is important not to oversand because you'll create a depression in the profile of the body. Airplane bodies have beautiful curves with C2 continuity that you don't want to ruin by putting a concavity in the shape.

It is important not to undersand because you'll ruin the curves the other way (convex). Hence the use of paper stuck to the finger to get the feel just right.

Repairing a small tear in the tissue paper on a balsa airplane

Small tears happen for a variety of reasons. In this post, we'll cover tension tears. A tension tears when the paper dries an pulls so tight that the paper tears. Tension tears are mostly easy to fix. The basic process is a lot like putting a small piece of toilet paper on the face (to help clot the bleeding) after cutting it shaving.

By hand, not with scissors, tear out a piece of tissue paper about 2x the size of the hole. Don't worry about getting the paper perfectly round or etc. The idea is that the paper edge will not be visible. If the tissue paper needs to be bigger than 1/2 inch square, then you should use the technique for large holes (we'll cover that later). Tearing the paper is very important because it will leave a nice mat of paper tissue fibers that will glue ever so perfectly to the old paper without leaving an edge. If you cut the paper with scissors, then the edge will be visible.

Get the new piece of paper wet and stick it on the hole (just like shaving!) Dab a little glue (I use Elmer's Gel school glue) on the patch and then, with a wet brush, gently spread the glue around. Getting the patch and the surrounding paper wet will look pretty bad. Don't worry, it will look good when it dries.

Glueing on the paper

This should be done after forming the leading and trailing edges and sanding the frame but I do it before glueing the wings to the body, usualy.

1. Lay the piece on the tissue paper and cut around the piece leaving about a 1/4 to 1/2 inch border. Its possible to trim the paper later, but it's a lot harder to make the paper bigger later.

2. Use a big (2-3 inch) brush to wet one edge of the tissue paper. I usually wet the edge that will connect to the shortest straight part of the piece to be papered. On wings, this is bit where the wing connects to the hull.

3. Put the wet edge on the piece, hold the other end of the paper with one hand and your brush in the other. Work your way from the wet end to the dry end using the brush to wet and stick the paper as you go. Try to avoid wrinkles but they aren't critical yet. Only the edges of the paper should be wet at this point. (If you get the middle wet now, then you might tear it in the next step)

4. Pick up parts of the paper that appear loose, wrinkled or out of place. Very Very gently tug on the paper (after lifting it a little) to straighten it out. It doesn't have to be very tight or perfectly straight at this point. The paper will shrink some as it dries. Be exceedingly careful not to tear the paper.

5. Use the brush to wet the rest of the paper. It might sag even more at this point. Very carefully lift and tug the paper to straighten in out.

6. I use Elmer's gel glue. I lay a bead down on the outer frame of the piece and then go in with a wet brush to spread the glue flat, but I do not spread the glue to any other pieces of wood on the piece. The idea is that I want the paper to be stationary relative to the edge of the piece but that I want it to pull tight in the middle. So if I glue the paper in the middle, then I will get funky stress lines as it pulls against the wood.

That's it. Repeat over and over until done.

I try to avoid paper-to-paper joints but that is not always possible.

Repairing a broken stringer

The stringers are those small balsa strips that your form the body of the airplane and wings with. The problem with stringers is that they are easy to break. I get lots of experience with this as I have 2 1/2 year old twins. Fortunately, they (the stringers that is, the twins aren't broken) are easy to fix too. See picture...

To fix a broken stringer, finish breaking the stringer and remove only the section of the broken stringer between the two adjecent former pieces (the former pieces are the cross section pieces that hold the stringers in the right places).

Next, carefully carve out the bits stringer glued to the former to half the width of the former. This leaves the rest of the stringer attached to the former while giving you a little notch to glue the new piece to. To do this, I insert the xacto knive parallel to the former and perpendicular to the stringer and push out a little wood. It takes about 1 minute of repeatind the process to get rid of all the wood.

Cut a new piece by taking a piece of stringer stock and laying it in the place it will go and cutting it to fit. Of course, make it a little too big. You can always trim it later but you can't make it 1/128th of an inch longer.

Finaly glue it in place et voila you have repaired a broken stringer.