|Apr 19, 2011, 07:53 AM|
Joined Jun 2006
working with silk, Minimaster
Working with silk.
Silk is a natural fabric It is woven and will cover around compound curves very well.
I've covered foam planes with silk, and balsa too.
This will be about covering open areas of a balsa structure or a solid sheeted area.
Minimaster build thread:
First prepping the wood.
Balsa is cellulose based and it absorbs colored dope/paint like a sponge.
The faster you can seal the pores of the balsa, the less coats of paint you will need.
Dope, both clear Nitrate and butyrate, fills the pores in balsa well and stops the wood from sucking up paint.
The better the finish of the uncovered wood, the better the overall finish will be.
A few coats of clear will seal and then start to fill the grain of balsa.
Some people add talc powder to the dope so it fills the grain a little faster.
Some use a product called sanding sealer to fill the grain rapidly.
I prefer thinned dope. Thinned dope fills pretty fast and doesn't get too thick over the high spots.
Then I dry sand with 180 to 220 grit sandpaper using a block to get all the high spots down.
The goal is to get an even smooth surface, before you cover.
A lot of people use nitrate dope for the first steps.
I feel butyrate works just as well.
But remember nitrate and butyrate are not the same.
Nitrate can't be applied over butyrate. It will react and ruin your finish.
Nitrate is very flammable compared to butyrate.
Nitrate has a little better adhesive qualities compared to butyrate.
Nitrate requires additional fuel proofing.If your plane is to be glow powered.
Nitrate stops shrinking after a couple of days.
Butyrate shrinks for a long, long time.
Since I don't use nitrate anymore, This will be using butyrate. The proceedure for Nitrate is the same.
To increase the adhesiveness of the butyrate I coat the bare structure with butyrate until the wood picks up a shine after the wood has been sanded smooth.
I like a 3/4 inch wide camels hair brush. These are available from hobby or craft shops.
Some people have had good luck with the foam brushes. I like the foam brushes for paints other than dope.
Applying silk to an open structure.
Lay a piece of the dry silk over the area to be covered.
Silk has a grain to it. The grain should go spanwise.
If you put the silk on so the grain is chordwise the silk will pull down between the ribs and look odd.
Silk shrinks greater with the grain than across the grain.
The grain is usually the side with the edge finished from the factory.
Wetting the silk with water.
I use a small atomizer to wet the silk.
Keep the silk wet, I mean really wet.
It will stay put just from friction if it's wet.
I use a lot of thinner with just a little dope to stick the silk.
The thinner passes through the silk and softens the dope underneath sticking the silk down.
If you accidentally get some of the thinned dope on the silk, there is not enough dope to curl the silk.
Thick dope will curl the silk as it drys.
Using dope while the silk is still wet will cause the dope to blush. It will actually turn white.
Let all the water dry off the silk before doping too much.
Avoid days with high humidity
If it is a day with high humidity, the dope may blush again. A couple of drops of retarder will stop that.
Sig sells retarder in small bottles.
A couple coats of dope on the silk over the structure helps to bond the silk to the balsa.
Do that before trying to fill the weave of the open areas.
There are many ways to stop dope from dripping through the silk.
I like the thinned dope method, it always works.
When filling the weave of the silk, use cross coats.
One coat chordwise followed by one coat spanwise.
I usually do this to each wing panel before going to the next panel.
Avoiding the drip through.
When doing open wing panels, just use very thin dope and hold the surface vertical.
If you hold a wing horizontal while doping, especially with dope that hasn't been thinned a lot, the dope will drip through and make an ugly ''ring'' on the other side of the wing.
The thinned dope will not leave a ring, and if the panel is vertical the dope will stay on the side you are doping.
The newer silks have a much tighter weave than the old silks. The new silk fills a lot faster.
If you've been using nitrate dope, this is a good spot to switch to butyrate.
On the last couple of coats of clear I add a couple of drops of Castor oil to the clear dope. It will act as a plasticizer and will slow dope from drying out over a long time.
I've found that there are at least two different silks that shrink different from each other.
The silk from Thai silk seems to be pre shrunk and doesn't shrink much if at all after covering.
This is good for delicate structures. Or over solid sheeted areas.
Non shrinking silk
The silk I like from there is the 5mm 021F-000 Habotai.
If you want silk that shrinks more, the silk from Dharma Trading is for you.
It's great for stronger structures or open areas like wings.
I use the 5mm. HS536 or the HS545 same but 45 inches wide.
I've found the 5mm silk is lighter than any other covering, including tissue and the weave is so tight that it doesn't take much clear to fill.
Before you ask,mm is not milimeters. ''MM is Momme.
Momme (pronounced ''mommy'' and abbreviated ''mm'') expresses the weight in pounds of a piece of material of size 45 inches by 100 yards. So, for example, a 50 yd. bolt of our 5mm 45'' Habotai Silk fabric (#HS545) would weigh 2.5 lbs. (plus the weight of the cardboard tube it is wrapped around, of course).
The higher the momme, the heavier and stronger the fabric. Anything above 28 momme is considered heavy-weight and generally used for curtains or heavier outer-garments. Silk under 20 momme is considered lightweight, and generally used for light blouses with a chiffon feel. Medium-weight silk (20 to 28 momme) is excellent for silk scarves, furnishings, wedding dresses and the ultimate luxury of silk sheets.'' Dharma Trading.
5 mm silk is extremely light.
A sheet 45 inches wide by 300 feet long would only weigh 5 pounds.
Drying out and rot.
Unless you leave the silk in direct sunlight, the silk won't rot.
What actually happens over time the dope becomes hard and brittle. The silk is saturated with the dope and when the dope cracks, so does the silk.
I just started removing silk that was applied 40 years ago. The silk is fine, the dope isn't.
If you plan on leaving your plane in the sunlight for storage, a couple of coats of silver will stop Ultra Violet damage.
Dope to thinner ratio, I find I use twice as much thinner as dope.
It seems that silk only comes in white lately. Colored silk over an open framework with just clear dope is a nice light finishing technique.
It is easy to dye silk.
Now to the color part.
I've been told ,"you can't dye silk without a special process".
Once way back when, I tried with the little packages of Rit dye.
I made a mess and the silk was all streaky and not colored evenly.
I read the process at Dharma and it basically told of a long and involved procedure to color silk.
My wife said,"try this". She handed me some new Rit liquid dye, and an old pot.
When I finally got up the courage to try it, I was amazed.
I didn't make a mess and the silk took the dye evenly through out.
All I did was follow the instructions on the back of the package.
Basically add the liquid to hot water and let simmer on the stove.
Add the wet silk and stir for thirty minutes.
Rinse with cold water and wash with some detergent, let dry.
The next day wash again with a mild soap or detergent. It seems the excess dye doesn't go away completely with the first wash.
Failure to do the second wash will result in the excess dye transferring to the clear dope by way of the brush and mixing together, and contaminating the clear.
Also when you go to add colored dope over the dyed silk, the color will bleed through to the surface.
Darker colors are harder to cover over.
Red is the worst.
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