Building the (Old) FLYLINE Great Lakes 2T-1A Kit (23)

Whether we call it “silk-and-dope”, “dope and tissue”, “polyspan and nitrate”, or whatever, covering and finishing model airplanes using a sheet of paper or fabric and then sealing it with various paint products involves two different operations. We have just finished the first, which is cutting, attaching and shrinking the covering material (in this case, polyspan). In many ways this is not so different from what you’d do with pre-finished film or fabric products, but the second part of the job is way different. Sealing the covering material so that it becomes airtight and smoothing it to provide the base of a slick finish requires a set of skills very different from what many new model builders get to learn using those well known products. That’s what we’re going to do now.

DOPE is at the heart of the matter. Ever since people began using it way back in the days around World War I there have been jokes about other kinds of dope that do strange things to your mind and, yes, I know all about the contention that anybody strange enough to shut himself up in a hangar or a room with that intensely aromatic concoction in order to paint it onto the fabric covering of an airplane probably doesn’t need the other kind anyway.

You probably already know that airplane dope is a kind of paint that is similar to lacquer, very light, and capable of shrinking energetically enough to pull whatever it is applied to very tight. There are two kinds. Butyrate dope (cellulose acetate butyrate) is the kind still used on full scale fabric covered airplanes and was pretty much the only choice for model builders from the late 1940’s up until MonoKote started the trend toward one step iron-on coverings in the 1960’s. Nitrate dope ( cellulose nitrate) was on hand before that, but it became less common in full scale use because it is more aggressively flammable than butyrate and, for model airplane purposes, exposure to alcohol based model glow fuel turns it into sticky useless gunk.

But…(and this is a big one)…nitrate gets its drying (and shrinking) done rapidly and predictably, while butyrate goes on just about forever getting harder and tighter (and maybe warping your beautiful new wing). Just about any other sort of resilient, fuel-resistant paint product you might think about using on a model airplane these days will adhere easily and tenaciously to a nitrate dope base, while just about anything other than more butyrate will tend to fall off a butyrate base. Of course, if you have discovered the wonders of electric powered flight the fuel part doesn’t matter.

Here’s where I get mine: http://www.freeflightmodels.com/

I’m going to use my favorite combination of finishing materials for smaller models like this one…nitrate clear dope and several materials from the Stits Polyfiber catalog, a line of products developed for full scale aircraft use that I have been using with outstanding results for the past twenty years or so. The Stits Polyfiber fabric is my first choice for larger models, but for little guys like this Great Lakes I prefer lighter materials such as silkspan or polyspan, sealed with nitrate clear dope and then finished with various Stits products with which it is entirely compatible.

Check out the Stits Process at: http://www.stits.com/

This is the left tip of the lower wing with the Polyspan fully attached and shrunk tight. At this point there is no clear dope on the covering. It is as smooth and wrinkle-free as it is ever going to get, but the addition of some clear nitrate dope will pull it just a bit tighter.This is a cabane strut assembly, fully covered with silkspan and sealed by the dope that was used to attach the covering. More dope will harden the surface somewhat and bring up a sheen.This is the first coat of clear nitrate dope going onto the upper wing. Notice that I am using a flat, wide, soft brush and applying the dope as generously as possible without allowing it to sag or run. The intent at this point of finishing the covering is to get the material sealed...airtight and free of pinholes...so we can move on to creating a finish base.

This is the upper wing center section trailing edge with the first coat of clear dope fully dry. You can see that the covering has begun to show a faint sheen and that it is stuck down really tight along the structural edges like the top of the ribs.

This is the tip of the horizontal stabilizer at the same stage as the wing center section. The entire surface has been given a generous coat of clear dope and allowed to dry. Dope can be deceptive...sometimes you can assume that it's dry before it really is, and adding successive coats over partially wet dope can cause uneven shrinkage and warped structure among other things. Nitrate is a lot more tolerant than butyrate is this respect. Ideally, let each coat of clear dry overnight before adding more. At very least, wait until the coat of dope you think is dry has completely stopped feeling cool to the touch from evaporation of solvents.

This is the toughest part...sanding doped covering. The simplest explanation of getting it right is to sand very gently (I am using 320 grit dry paper here) until the faint fuzz is gone from the surface. If you sand too much, or too aggressively, you will cut through the covering along a structural edge like these fuselage stringers and have to start over. If you don't sand enough you will end up with a less than perfect finish and extra weight left on the airplane. There is no way I can TELL you how to get this part right...you have to DO IT, over and over, until you develop a sense of what working with dope and fabric feels like and establish your own standards.

I have combined a few steps here. On this model I used two coats of clear nitrate dope to get the covering sealed and provide a base for sanding that would get rid of the intial fuzziness. I followed that with two airbrushed coats of Stits Feather Coat primer, which is what you see being applied to the top engine cowl.. I'm not going to offer a course in airbrushing/spray painting here...there are plenty of better places for you to learn that...but I will say that if you are interested in this aspect of building model airplanes, these are skills you can't do without.

This is what the first coat of Stits Feather Coat primer looks like just after I finished spraying it on. The material is still wet and has a pronounced shine. The trick in building up a primer base is to get as much material as possible onto the surface without allowing it to sag or run, so that when it is dry you have the maximum possible sanding base to work on. Notice that I have propped both sections of the cowl on simple jigs so they can dry without the wet primer touching any other surfaces.

This is the fuselage side with the first coat of Stits primer fully dry, ready for sanding. As you progress from the first dope coat through the last application of primer, sanding everything as you go along, the surface of the airplane will get progressively smoother and shinier.

This is the top wing center section...the same structure we looked at before with a single coat of clear dope on it...with the first primer coat dry and ready for sanding.

Same game with the wing tip. Part of the learning process is to learn what to look for as successive coats of dope and primer go on...you need to know which bumps and fuzzies can be sanded away and which will need to be sprayed with more primer before you'll have enough sanding base to get the job done.

This is the horizontal tail at the same stage of priming/sanding as the wing. It's looking good, but there are still imperfections that can be removed.

We're back at the wing center section with the Stits primer sanded as aggresively as I dared. You can see where I have removed what were once high spots right down to the clear dope/fabric substrate. This is the part that demands serious practice for you to get it right. Sand too deep and you cut through into fuzzy covering or balsa and have to fix the mistake. Sand too little and the high spots don't go away.

Sanding primer...this is the BIG SECRET do doing paint and fabric model building and getting it right. There is no substitute for experience and the only way you can get it is by doing it. As you can see, it wil get messy and I'll guarantee that your fingers will get tired before the finish base looks the way you think it should. Knowing what to do RIGHT HERE is what makes master model builders.