Airfield Models - How To

How to Silk a Model Airplane Wing

July 25, 2009



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Airfield Models (http://www.airfieldmodels.com/)Sealing Balsa Wood in Preparation for Silk

This step is extremely important.  If you skip it or don't get it right then all kinds of bad things can happen including the structure warping, buckled sheeting or the silk pulled into the balsa wood grain which isn't pretty.

It seems like all my articles start with, 'The most important part of successfully <doing whatever> is preparation.'  That's probably because the most important part of successfully doing whatever is preparation.

The most important part of silking a model airplane wing is preparation.  Any finish should begin with a good base.  In this case that means a completed wing that is finish sanded using 400 grit garnet sandpaper on a good sanding block.

My wing cradle made the wing a lot easier to work with during the entire finishing process including finish sanding.

  • Sight the wing to ensure ribs flow evenly from one to the next.  Sand as necessary.
  • Ensure all sheeting is smooth, free of scratches and any seams are flush.  Sand as necessary.

  • Sight the leading and trailing edges from both tips.  The leading edge should have a consistent and correct contour for the entire length.  Sand as necessary.

  • The trailing edge should be straight.  Sand as necessary.

  • The leading edge, trailing edge, wing tips and sheeting should flow perfectly into the rest of the wing.  Sand as necessary.

  • Vacuum the wing with a good brush attachment.  Use the vacuum to pull dust and crud out of the brush before attaching it to the hose.  Inspect the brush carefully and run your fingers through it.  If there is anything in the brush that can mar the wing then don't use it.

Model aircraft dope.The wing must be well sealed with clear dope anywhere the silk will contact it.  In my case that meant the leading and trailing edges, wing tips, center section sheeting and all cap strips.

What we are trying to achieve is an outer coating that is water proof.  The silk stays wet for quite some time.  if the wood isn't sealed then any and all of the bad things listed at the beginning of this article can happen.  The dope used to apply the covering permanently seals these bad things into the finish.

In case you're thinking you can avoid all this by applying dry silk to bare wood then you should know that wet dope will swell the wood slightly and cause some of the same problems, particularly the silk being pulled into the wood grain.  The wood must be sealed whether you apply the silk wet or dry.

Silk is much easier to manipulate over a non-porous surface.  If you've never played with wet silk before then all you need to know is that it's the most clingy stuff imaginable.  It takes a good amount of effort to make it evenly taut with the fabric grain correctly aligned.  Pulling it over sealed wood is a lot easier than unsealed wood.

 
 

Tools and Supplies Needed

  • A good quality charcoal mask made by 3M.Excellent ventilation.

    Dope is extremely toxic.  Imagine having a big jar of fingernail polish open under your nose for a couple hours.  A charcoal mask is a very good idea.

  • Clear Nitrate or Butyrate dope.

    Some people say that nitrate dope has better adhesive qualities than butyrate.  I don't know if that's true or not but I've had no problems using butyrate for the entire job.  Nitrate isn't fuel proof so if your plane is glow powered then you should use butyrate dope exclusively after the covering is attached.

    Butyrate dope can be applied over nitrate but nitrate can not go over butyrate.  Once you being using butyrate you must continue with it.

  • Dope thinner.

  • A Jar having a lid with a liner that isn't affected by dope.

    Dope solvent is very hot and will eat through foam or rubber cap liners from food jars.  If you can't find a jar having an appropriate cap then you can always put a piece of aluminum foil over the jar before screwing on the lid.

  • Flat 1/2" and 1" natural hair paint brushes or an airbrush with a large nozzle.

  • Acetone for clean up.

  • Paper towels.

  • Sanding block with very fine paper.

Dope is much too thick as it comes and must be thinned to brush well.  Find a jar with a cap having a liner that the dope won't melt.  An 8 ounce jar is large enough to not need constant refills but small enough to work with.

Fill the jar about 1/3 full of dope.  Thin it until it's a good brushing consistency.  I usually start with an equal quantity of thinner and adjust from there.  If it's too thin then add more dope.  If too thick then add more thinner.  Don't use retarder in the sealing coats.

Sealing the Wing

An airbrush with a large nozzle can be used to apply sealer coats and will save you a lot of time.  It may require several more coats as the coats will go on thinner than if you brush.  A Paasche Type H with the large tip is perfect for this.  Keep in mind that an airbrush is the wrong tool to dope the overall wing.  See the linked page for an explanation.

  • Only use natural hair brushes to brush dope.  Synthetic brushes work poorly when applying dopes and lacquers.
  • All sealing coats are brushed on one side of the wing.  When the first side is dry the other side is coated.  Three brushed coats are usually all that are necessary.

  • Exposed end grain wood should have additional coats added any time you're adding any other coat.  For example, if the wing tip has end grain then coat it when you coat one side of the wing and then coat it again when you coat the other side of the wing.

  • Seal any hatch compartments, holes or anything else water or fuel may be able to work its way into.  If your wing has center section sheeting then it may draw water into the end grain from the wet silk.  Run the brush around exposed end grain and holes that the silk covers with each coat.

If the wing ribs are not capped or sheeted then doping them (or getting them wet) will raise the wood grain.  What that means is that if you dope silk over unsealed ribs there will be a lot of ugly bumps under the silk that are there forever.  Be sure to seal the edges of the ribs well.

Lightly sand the final sealing coat.Sand lightly between coats to remove any raised grain and make everything smooth.  Sand as little as possible so that you aren't sanding back down to bare wood.

Keep applying thin coats of dope until the grain doesn't raise.  Now apply a heavier coat of dope that is thinned to brushing consistency.  This dope should not be as thin as the dope you've been using up to now.  Sand after this coat.  Keep applying these coats until you can sand without exposing bare wood.

I normally need to brush on two coats of very thin dope and then one or two coats of normally thinned dope.

Don't get stuck on the number of coats.  If you apply thinner coats than I do you may need more.  It's not the number of coats that's important.  It's that the wood is actually sealed and wood grain isn't raised.  If you can't tell if an area is sealed then run a wet finger over it.  If the wood grain raises or the wood feels slightly mushy then it's not sealed.  Allow the wood to dry, sand it lightly and recoat.

You can apply a final coat if you like.  It won't hurt anything and clear dope weighs practically nothing.

Vacuum the wing when you're satisfied with the sealing coats.  If you have an air compressor then blow out the wing and vacuum it again.

The wing is now ready to silk as soon as its mounted on something that prevents the wing from moving around and allows the silk to hang straight down all around the wing.

 
 

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Introduction to How to Silk a Model Aircraft Wing
Apply Silk to a Model Aircraft Wing

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Copyright 2009 Paul K. Johnson