One of the modelers on my distribution list asked
me why I double cover silk over Polyspan Lite. Here is the answer I provided
him. I believe there are three good reasons to double covering silk
over Polyspan Lite:
(1) It extends the life of the silk
covering almost indefinitely and prevents the silk from ever crazing with
(2) Silk over Polyspan Lite is probably
three times stronger than just silk itself, it adds very little additional
weight, requires fewer coats of clear dope, and there is no evidence that
the Polyspan Lite covering is even under the silk.
(3) Poylspan Lite does not have the
problem of dope seeping through the covering and globing up on the
underneath side with the first couple of coats of clear dope like silk does.
By covering with Polyspan Lite first with three coats of dope, you have a
base on which to lay the silk. Once you over cover the Polyspan Lite with
silk and apply one coat of clear dope, the silk fills almost immediately
with absolutely no globing up on the underneath side of the silk.
This afternoon I finished covering the
Sailplane's fin, rudder, and sub rudder with orange silk over Polyspan Lite.
the first coat of clear dope was without retarder to set the silk on the
Polyspan doped surface. Then I applied two more coats of clear dope with
retarder in it as shown below. The results look really good to me.
Wallock Tip: Keep some Acetone in a small covered jar for
cleaning out your brush periodically when doping. The Acetone is fast acting
and removes the dope quickly. However you must keep the jar covered
because the Acetone evaporates rapidly. Also, the Acetone is great for
removing the clear dope build up on your fingers from when you rub in the
dope on the edges with your fingers while covering.
Here is close up of the fin's front fillet
covered with orange silk over Polyspan Lite.
I also want to point one other covered area. I
covered the 2-56 threaded plywood inserts on the bottom of the fin with
white silk for additional reinforcement. I did not cover the fin's complete
"plug" that fits down into the stab slot in order to prevent the fit from
binding. The covering is really not required because as you can see, the
fore and aft spruce supports extend down to the bottom of the plug.
I took this picture especially for Thomas Ryan.
He was concerned about the rudder tiller's brass tube bearing CA'd to the
trailing edge of the fin in the beginning. As you can see in the picture
below, balsa was filled in on either side of the brass tube. Two layers of
Polyspan Lite covers the area, and two layers of silk covers the area. The
brass tube is well fixed to the trailing edge of the fin.
I still have the sub fins that attach to
the bottom of the stab to go. They are not even covered with Polyspan Lite
yet. I have been ignoring them because of the wire protector that has to go
on the bottom edge. However, I have started start work on doping and sanding
them and I hope to get them covered tomorrow....................Tandy