The Return of the TigerKitten (13)

During one of my last building sessions I got the entire airplane covered and the Polyspan covering heat-tautened uniformly smooth and tight. During that part of the process of covering and finishing Polyspan is a bit like the popular one step iron-on film coverings. (If we had been using a traditional covering material like silkspan, tissue, or real silk, I’d have used clear dope as an adhesive and relied on wetting the covering with water to make it shrink.) I like to think of Polyspan as synthetic silkspan, and at this point the differences between it and the older covering material goes away. On this building job I am going to use clear nitrate dope to fill, seal and strengthen the covering, then a sanding primer (in this case, my own home-brew made by adding talc…baby powder…to clear dope in whatever proportion I need to make more or less thick and sandable.)

The purpose of the clear dope is to seal the covering; the purpose of the sanding primer is to build up a base coat that can be sanded as smooth as I want to make it as a substrate for my color finish. When I’ve gotten to that point I’ll spray on just enough Stits Lite Coat white primer to provide a uniform white base for the color that will follow.

I'm using a wide, flat, soft brush to put the first coat of clear dope onto the bare covering. At this point any fabric or paper covering is going to soak up dope like a dry sponge. What you need to do is to brush on a generous wet coat of dope that's as heavy as you can manage without letting the wet dope sag or run...you want to get every square inch of covering saturated .

Dope...especially nitrate dope...dries fast, but not always as fast as you might think. Usually this takes a couple of hours before the coat of dope is dry enough for light sanding. The doped surface may feel firm and dry before it really is. If you can touch the doped covering and feel any sensation of coolness (from evaporating solvents), it's still NOT dry. Wait. When it is dry, I use 320 grit production paper very lightly to cut off the tiny "fuzzies" that appear on the covering. DON'T press hard enough to cut into the fibers of the covering material.

Here's the rear fuselage and fixed tail surfaces after one coat of dope and sanding as I just described. This is the point at which a doped tissue or fabric covered model begins to come alive. The doped covering bonds to every part of the airplane it touches and the whole thing becomes a taut, crisply vibrant structure, with an air of authority that no film covering will ever come close to matching. It's got soul.

Once the first coat of dope has been sanded out, I add the second. You can't see it, but at this point I'm switching from ordinary (shrinking) clear dope to non-tautening dope...the covering is now as tight as I want it to be. Shrinking further would threaten to distort and twist the structure, which is not good.

This is how the Polyspan covering looks after the second coat of clear nitrate dope has dried. The difference in appearance from the first coat is subtle, but it's there and you can learn to recognize it.

I'm giving the second coat a light sanding with that same 320 grit production paper. Most of the "fuzzies" that appeared after the first coat dried are gone now and the surface I am sanding feels smoother. However, it's not fully sealed yet, so it's still too soon to let the paper begin to cut into the surface.

More subtle stuff...I've given the wing its third coat, this time with my dope/talc mixture. For places like this, I mix my sanding primer with enough talc to give it the consistency of heavy cream.

Now I am beginning to sand aggressively enough to cut into the surface of the dope/talc coat. If you look carefully you can see loose sanding dust (talc) on the wing surface. Knowing exactly how much to sand, how hard to press, is a set of skills you can only develop over time, with practice. It it worth the effort? Take a look at some of the other airplanes I have presented here on rcmodel.com and YOU decide.

I put two wet coats of dope/talc sealer onto the entire airplane, sanding meticulously after each coat dried. Now I have switched over to Stits Lite Coat primer...that white stuff...to get a totally opaque surface. This accomplishes two things...it makes spotting whatever flaws and blemishes are still there so they can be given extra attention with the sandpaper, and it will provide a uniform base that requires a minimum of Stits Polytone (color) to cover evenly. (Color costs more and weighs more, which is why we use as little of it as possible.)

It's judgment call time. When the first shot of Lite Coat dried, it permitted me to see tiny bumps and wrinkles that weren't visible before. I mixed up a small batch of really thick dope/talc to use as a spot putty and dabbed it onto each of the problem spots with my pallette knife. Here you see it still wet as I'm applying it to the horizontal stabilizer. Be careful here... adding a surface filler like this over a fabric covering demands a fine touch. Nearly everything I'm putting ON here will have to be sanded OFF without permitting the sandpaper to cut into the covering. That's going to take a really delicate touch to pull off, and it's something you'll want to consider carefully before you do it on YOUR model for the first time.

This is the same dope/talc filler on a bad spot I found on the wing, after it has dried.

I'm using one of my long, straight sanding blocks with 320 grit production paper to cut the spot of filler on the wing down even with the surrounding surface.

Same deal with the horizontal stabilizer. This is where it pays off to learn to sand with a slow, gentle touch.

The final smoothing and blending of the last little traces of an edge on the patched area is done with more 320 grit and calibrated fingertips. In fact, you can very often FEEL an irregularity at this stage of a finish job when it's difficult to SEE it.

Sanding out the Stits primer coat surface on the underside of the wing center section. You can see the dust on the work surface and on me. Surprisingly, the most common error at this point is not sanding too aggressively as you might suppose...it's not sanding PATIENTLY enough and leaving extra primer, along with visible roughness and extra weight, on the surface of the airplane.

Here's another look at the way Stits Lite Coat primer should appear after you've sanded it properly. I have sanded into the primer coat as far as I can without cutting through the final bit of dope over the covering, and the primer is GONE from what used to be the high spots.

Another look. This is the horizontal stabilizer after I've finished sanding out those little patched spots.

This is a wingtip after I sprayed more of the Lite Coat over the fully sanded wing. You can see how smooth the entire surface has become, and how the uniformly white surface is going to be easy to cover with a minimum of color.

I'm sanding...very gently and very selectively...one last time before it's time to spray color. It's also time for another judgment call. At this point the base coat surface is GOOD, but it's not PERFECT. I could go through two or three more cycles of spray-dry-sand, getting down to 400 grit or even finer wet-or-dry paper, but I'm not going to. This TigerKitten is a club field, Saturday afternoon sport airplane, not a candidate for concours honors. I could get a better finish, but it would be at the expense of several additional ounces of weight. Like a lot of other activities, the trick to success here can be knowing when to stop.

One last look. Here's the horizontal stabilizer again after I've just brushed over it with the 320 grit paper. Now that they look like this, I'll dust ALL the surfaces off, give them a light wipe with a paper towel and denatured alcohol, and consider them READY FOR COLOR.