Another Silkspan Question

"Z"

A friend of mine is coaching me through the process of covering open bay wings with silkspan. I have read almost everything I can find on this subject and felt pretty comfortable with the process. I was always under the impression that after the silkspan is installed, the next few coats should be brushed on. In our conversation he mentioned that he generally sprays on the next few coats. What is the benefit of each method and does it really matter.

Thanks,
"Z"

With some covering materials, it's fairly easy to get runs started on the "back" side when using a brush, thus, you'll see some comments about holding a wing in the air and sort of doing the application "from underneath." This gets difficult with the bigger planes if you don't have a large FIXTURE. Those were sold for awhile; kinda expensive in a way, but let you position a model in just about any attitude without worrying about getting any fingerprints in wet paint!

I don't recall seeing that happen with medium or heavy silkspan - nor with the original grades of Jap Tissue. It's a serious deal to work around with silk (or any other fabric that isn't pre- finished), and I do remember problems with LIGHT silkspan.

Using a spray rig is a quick way around the runs with silk!

Kiwi

I have heard from several (though NOT Windy, the Bagwan of ProStunt) that mixing up some clear Knox geletan and brushing it on, then letting it dry, then brushing clear or whatever onto the silk or silkspan would hold the dope to the outside and prevent those unsightly drips. When totally dry, lightly tapping the wing realeses the gel (now solid/powder) and tilting the wing yon akimbo allows the stuff to run out from where the leadouts emerge.

The guy who advocated this was a champion builder, although on the scale side. Of course, the same guy recommended covering the whole fuselage with Bondo, too.

Try a test patch... I'll bet that it works!

See Ya

Something I picked up from freeflighters a few years ago about getting clear dope onto silk (I have used this technique very successfully with silkspan, too) solves all the drip-through worries of the first few coats of dope on silk/silkspan:

Get a roll of toilet paper. Keep it handy. Now, starting a an edge somewhere, like center of the wing, or the fuselage joint if the wing is already fitted, lay your toilet paper strip 3-4 sheets long down on the wing. Get your brush well loaded with dope, and apply it to the toilet paper, getting a good wet patch on the toilet paper the whole width of the paper. Drag the paper along the wing, the dope will transfer from toilet paper to silkspan. Keep dragging and wetting the toilet paper, keeping that wet spot in complete contact with the wing. Work your way to the tip, lay the paper down next to the doped area, and do it again. If the paper tears or gets worn, ditch it for a new piece and start again. Using this technique, I never have had a drip go through silk or silkspan, and I have a nice yellow-silk-covered Waco cabin biplane as evidence.

This technique should be done for the first couple of coats, then you can brush on the remaining coats you wish once the pores are sealed up and no drip-through can happen.

You might also apply the dope using a foam brush. Got that idea from George Aldrich. Works well.
By the way, we all know to apply the silkspan with the grain running spanwise, correct?
George

Regarding grain direction, tissues have grain direction and so do some of the synthetics, like (Super?) Coverite. I'm not sure how to test that product to determine grain direction, but it definitely does. I'd bet that the directions tell you, so it may be worth a read. Spanwise always, unless double covering, in which case first layer spanwise, second layer chordwise. Steve

Determing grain is not all that difficult. God made organic fibers that get fatter, but not much longer, as they gain moisture. Grain is a man-made thing that says more of the fibers are aligned North-South vs East-West.

Take a paper covered book you are not particularly fond of and cut a one inch square out of the cover. Lick the back side of it. It will curl so the axis of the curl is parallel to the spine of the book.

Any man made product from organic fibers has grain, and the shrinkage and growth along the grain direction is always less than the cross grain growth / shrinkage.

For silkspan, silk, or any organic material, we can find grain direction and effect by cutting a sample and measuring it. Then wet it and let it dry. Compare the dry measurements to the pre-cut measurements and you will have a picture of the grain direction and how "grainy" that product is.

Generally you can see the grain by holding the silk or silkspan up with light coming from behind. Sig Plyspan, in my experience, has the grain running parallel to the short side of the sheet.

I've heard of the toiletpaper trick but have not tried. Brushing with dope thinned down 70% for the first two or three coats will help with the run problem.