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