> If anyone is 'up' on Polyspan, I would
> like to know if it MUST be used with Nitrate dope.
> Anyone tried Butyrate with this product?
Took me a while to figure out polyspan, but I'm lovin' it so far.
I get mine from Model Research Labs an outfit with a rather basic website but a lot of experience in materials and coverings. They are oriented towards free flight but the benefits of polyspan apply to stunt. MRL sells a particularly thin brand of polyspan. Given how tough the stuff is there is no point to put heavier versions on an airframe.
They have a page full of instructions which are helpful to get started, but here's what I've learned the hard way:
A) Use something else (silkspan, light carbon veil, whatever) on the fuselage and the wingtips.
B) Repeatedly dope the outer parts of the wing structure with thinned dope and sand until you build a firm base of dope down into the wood. Put a final coat on and do your covering as soon as it is dry but before it gasses off too much. would not hurt to run a vacuum brush head across the final surface and get everything of that is not flat, smooth structure.
C) There is a glossy side and a matte side to the Polyspan (From MRL) and you want the glossy side out at all costs. This is easy if you remember that the materal is wound around the cardboard tube matte side down, glossy side out. It's ##### near impossible to tell the difference, so I just take the spool in my hand pull off enough to cover the wing panel and lay it on the wing with the side of material inside the spool face down on the wing. You don't want the bad side up!
D) There is a very faint grain to the material, which you want to run span wise to minimize droop between the ribs. This is easy if you remember the grain runs the length of the material wound around the spool. So just grab the cardboard spool and hold it over one wingtip and run out a length to go to the other wingtip or the center as the case may be. Now you have the bad side down and the grain running spanwise.
E) Cut away any excess while the material is draped over the wing frame. If you have tapered wings, cut a slice in the center and work gently with the material until it drapes smoothly over the structure on its own, then support the wing frame so that the material can lay quietly while you attach it. Drape the stuff right off the roll and then smooth out all wrinkles caused by the form of the wing. You don't want to be fooling around with the material while it's wet with dope.
F) Use a inch and a half brush or so and a batch of 50/50 thinner and clear dope. To answer your actual question, I have used nothing but Aerogloss butyrate on the three wings I've covered thus far. If you have a straight trailing edge, tack one end down and then use the brush to 'paint' the polyspan flat as you lay dope/thinner down just around the very edges of the panel or wing you are covering. DON"T start rubbing the material with your finger like we used to do with silkspan, you's just destroy the smooth surface and raise the dreaded fuzzies.
G) Take a couple of laps around the material easing the thin dope on to loosen the base dope and fill up into the polyspan from the bottom.
H) Wait until that dries and then do it again.
I) Wait until that dries and then turn the wing over and cover it. Leave the center of the materal a little loose as there's nothing to worry about. What you want is an even cover so that when things start to shrink they don't induce warps.
J) Wait until both sides are dry all around with minimal working of the edges where they overlap or whatever before you shrink the material. I use a hot air gun which is a small version of a hair drier. Set it on medium and start waving it at the loose cover a couple feet a way and then sneak up until you see the material begin to shrink. Which it will, just like magic. Run the hot air around in a circle from the center outward, evenly pulling shrinking all the material. It won't go drum tight, but it will shrink and snug up against the ribs. Be careful with the heat on the thin stuff, it's easy to burn a little hole. If that happens, just note the hole and press on.
K) Once both sides are shrunk, and the dope holding it in place is good and dry, then start in again with a fat brush full of 50/50 dope/thinner and just use the brush to flow the dope onto the polyspan. Lay the brush down flat on its side against the covering and let gravity flow the dope out of the brush into the poly, through it and into the wood under. Be patient and just lay the dope on slowly.
L) Let it dry and repeat as many times as you wish, at least until you fill the polyspan. After the second coat, find any holes you may have caused during the heating and cut a small circular patch 1/2 inch or so larger than the hole. Lay down some dope around the hole, make sure you put the patch matte side down (glossy side up) over the hole and lay down another coat of dope. Later on you will be hard pressed to tell where the patch is.
M) I put on a couple of thinned coats and let a couple of days goe by for it to gas off and then do the same over again, not sanding the surface until it's filled with clear dope and glossy looking.
N) When you have your base of clear on and it has dried real well, you can sand a little with medium to fine paper to get rid of any ridges. You do not want to sand through the clear into the material or you will raise up the dreaded fuzzies. If you get the polyspan material fibers sticking up, it can turn into a mess. The best solution to that is to put a little thin CA on them and then sand what is sticking up off.
That is as far as I have gotten, the rest of the alphabet is for you to fill in.
I plan a thin coat of silver, some sanding, my light colored dope and then dark trim. with a final coat or two of clear.
I'm not going for a concours finish.
But a half dozen coats of thin Aerogloss over the thin heat shrunk Polyspan has resulted in a translucent white covering that seems every bit as strong as Monocote and probably just as light, only I can now dope my color scheme on.
Aerogloss will shrink the remaining tauntness into the Polyspan, so you don't have to get too carried away with the heat gun.
I've done the tips of two planes in Polyspan, but it took a lot of little sections overlapped to do so, and I will not bother with that drill again, since silkspan does such a fine job around corners if you apply it right.
But Polyspan on a well build wing structure seems nearly bullet proof and quite light.