Applying Polyspan with Nitrate Dope.

[Mike Griffin]

COVERING OPEN BAYS WITH POLYSPAN:

Where to buy:  Two sources I can think of right off the top of my head are RSM and Tom Morris.

How to cover and dope on Polyspan on open bays:

The method I use is one that I learned from Charlie Reeves and it has always turned out great for me.

Put at least 3 coats of Nitrate dope on the airframe.  I use a 50/50 ratio of dope and nitrate thinner for this.  I usually wait a day between each coat.  After the third coat has dried, I sand with 220 grit sandpaper to smooth the balsa some.

NOW:    BEORE YOU DO ANYTHING, MAKE SURE YOU HAVE THE RIGHT SIDE UP BEFORE YOU DOPE DOWN THE POLYSPAN.

Polyspan usually comes rolled on a cardboard tube and I know Tom Morris used to put a red sticker dot on the side of the material that goes up.  It is difficult to determine which is the “up side” of Polyspan in bad light.  The side that goes up has a brighter sheen than the bottom side.  I will explain a little later why it is very important that you not dope down Polyspan with the wrong side up.

Anyway, after you have determined which side is up, measure and cut the Polyspan to size to cover one side of the wing.  I always cut the inboard part of the Polyspan long enough that I get a full double overlap on the center sheeting.  This adds a lot of strength to the wing. 

Take some nitrate thinner and a brush and after laying the Polyspan panel over the wing, start doping the covering to the perimeter of the wing.  This is much like you would apply your silkspan but Polyspan goes on dry.  Use the method you like to dope around the edges of the wing and keeping the wrinkles out of the Polyspan as you are doing the edges.  Round wing tips are more difficult to cover with Polyspan than silkspan so what I usually do is apply the wingtips separately or use silkspan for the tips.  Either way is ok.  I make multiple cuts in the covering around the tip to avoid as many wrinkles as I can.

Apply the Polyspan this way to all 4 surfaces of the wing.  Once you are satisfied with the covering job, take your heat gun and start shrinking the Polyspan.  Keep your gun moving and a few inches away from the surface of the Polyspan and shrink each panel until the covering is tight and the wrinkles are gone.

Once the Polyspan is down and tight, I take a brush and apply the first Coat of Nitrate dope straight out of the can.  I do not cut it with thinner.  I let this dry overnight.  The next day, I cut the Nitrate dope 50/50 with Nitrate thinner and apply another coat.  Keep repeating this process with the 50/50 blend for at least 3 more coats giving it overnight to dry and gas off.

After the 4th coat, I sand it (lightly) with 220 or 320 grit sandpaper.  Be very very careful about sanding on the rib tops or edges of cap strips.  It is very easy to sand through the material

I then take the 50/50 mixture of Nitrate dope and thinner and add a lot of zinc stearate powder or cornstarch to the mixture in a jar and then put the lid on and shake the daylights out of it till you have slurry.

Brush at least 4 coats of the slurry on the wing and open bays letting dry a day between coats and use 600 grit to sand between coats.

When you finish you should have at least 8 coats of dope on the surface and the surface should be shiny and look a little like an ice skating rink

From here it depends on how you are going to apply color.  If I am using something Like Rustoleum for color, I usually apply a coat of Rustoleum gray primer and then sand it mostly all off before I shoot the color.

Others who are shooting dope finishes might spray polar gray dope primer and silver and then spray their colors whether it be dope or auto finish paint.  Just depends on which method you like to apply your finish.

NOTE:  In case you do dope down the Polyspan with the WRONG side up, you will know it immediately because your surface will have a bad case of the fuzzies but not go jump off the cliff yet.  Take some very fine grit sandpaper (6-800 grit) and WET sand the surface, let dry and then recoat with 50/50 nitrate dope, let dry, wet sand again…. And keep this process up until you have a smooth surface.  I takes a while, I know from experience, but you will get it smooth if you are patient.  You might even want to use a finer grit paper if you have the patience.

Well hope this helps.  I am not saying this is the absolute best way to do it but it worked for Charlie Reeves and it works for me.

Mike

[Bill Little]

Thanks for the tutorial, Mike.  Sounds almost exactly how I have been doing it.

BIG Bear
RNMM/AMM

[Mike Griffin]

Well I had noticed Bill that there have been a lot of questions about it lately and this is something I know something about.  I have been using Polyspan for years and it is my favorite of all coverings.

[john e. holliday]

A heat gun  will help get the wing tips down and smooth or even an iron.  Be careful as it will tear if you try too hard.   Then dope the tips.

[Allan Perret]

COVERING OPEN BAYS WITH POLYSPAN:
Put at least 3 coats of Nitrate dope on the airframe.  I use a 50/50 ratio of dope and nitrate thinner for this.  I usually wait a day between each coat.  After the third coat has dried, I sand with 220 grit sandpaper to smooth the balsa some.

I do all 3 coats the same day, in fact I have done them with only an hour drying time between coats.  You can put on the 3 coats by 10am,  sand right after lunch, and start applying the covering the same afternoon.

[Motorman]

Reviving this old thread to ask, after you stick the polyspan on with the thinner how long do you wait to heat shrink it?


Thanks,
MM

[Clint Ormosen]

Reviving this old thread to ask, after you stick the polyspan on with the thinner how long do you wait to heat shrink it?


Thanks,
MM

As soon as it's dry. Here in low humidity California, I can heat shrink it an hour after sticking it down. Sometimes I'll wait to shrink it until after I get a good coat or two of dope on the edges.

[Mike Griffin]

Hi Walter, Clint is exactly right...

Mike

[Paul Walker]

I use about 4 coats of tautening clear on the bare polyspan in the open bays. This makes a huge difference.  I have several planes that are 5 years old now with this process, and they didn't become a pretzel.

Pet e Peterson put me on to this!

[Larry Fernandez]

Can Polyspan be dyed?

Larry, Buttafucco Stunt Team

[Motorman]

Can Polyspan be dyed?

Yes it can.  http://www.modelflight.com/larrydavidson.html

I intend to use as little dope as I can because, I'll be covering the polyspan with Coverite microlite. What I want to know is after you stick it down with the thinner do you need to put additional coats of nitrate on the frame to secure adhesion?

Thanks,
MM

[Clint Ormosen]

Yes it can.  http://www.modelflight.com/larrydavidson.html

I intend to use as little dope as I can because, I'll be covering the polyspan with Coverite microlite. What I want to know is after you stick it down with the thinner do you need to put additional coats of nitrate on the frame to secure adhesion?

Thanks,
MM

I usually do. Don't know if it's absolutely necessary, but I do it for insurance. I don't want it pulling lose during the shrink.

[Bootlegger]

 Is there a certain way to apply the p/span to keep from getting wrinkles/puckers at the cap strips where they join the l/e and t/e sheeting?
 Seems that I always get these after the dope s dry...

[Clint Ormosen]

Is there a certain way to apply the p/span to keep from getting wrinkles/puckers at the cap strips where they join the l/e and t/e sheeting?
 Seems that I always get these after the dope s dry...

Hmm... Never had this problem with Polyspan. Is it puckering so badly that a little heat won't remove them? I guess my only suggestion for this is to pull it a tighter chordwise while the dope is still wet.

You don't have any gaps between the cap strips and sheeting, do you? Even little ones might cause this.

[john e. holliday]

I put scraps of balsa in the corners and then round them out.   If you see some of the pictures the center section sheeting usually does this.

[Mike Griffin]

Is there a certain way to apply the p/span to keep from getting wrinkles/puckers at the cap strips where they join the l/e and t/e sheeting?
 Seems that I always get these after the dope s dry...

Hi Gil,

I have not had this particular problem and really can't think what may be causing it.  I would think if they were small wrinkles that the heat would tighten them up.

Mike

[RC Storick]

I went pretty in depth about Polyspan. Its a good method that yields perfect results.

Video with over 5000 views

[Fredvon4]

Back as a kid dad and I did silk and dope coverings and of course tissue too

When I found this site I reviewed Mike's instructions and later Sparkie's video and both helped a lot

I have relearned a lot from these guys and as much from many other contributors to SH

you all have made my retirement hobby so much more productive and enjoyable....but I still hate to sand.....grin

[Mike Griffin]

Since Silkspan as we once knew it does not exist anymore, the next best alternative is Polyspan.  This is not to take anything away from silk or iron on coverings but Polyspan is a great alternative to the old Silkspan.  Personally I like it better because you don't have to wet it and worry about tearing it or having it fold over on itself while handling it.  I, like everyone else, was forced into switching to Polyspan when Silkspan started disappearing.  I remember it actually seemed strange not to wet it before I laid it over the wing but it did not take long to fall in love with the stuff.

In my original post, I wrote about always making sure you put the shiny side up when you apply it.  Sometimes it can be a little difficult to determine the shiny side in certain light situations.  Here is another little trick you can use to make sure you have the right side up if your eyes play tricks on you. 

Squeeze a little Elmers white glue on your finger and smear it around a little between your finger and thumb and let it dry.  Then rub your finger with the dried glue over the surface of the Polysoan.  If it snags the material, you are feeling the downside of the Polyspan.  Flip it over and do the same thing and you will find your finger will glide over the material .  This is the up side.

Hope this helps.

Mike

[john e. holliday]

Thanks Mike, I never thought of that.

[Mike Griffin]

Thanks Mike, I never thought of that.

You are welcome Doc.

Mike