Plasticized dope mixture
 

I am presently covering a plane with silkspan and dope and was wondering the proper ratios for making a plasticized dope mixture . . .

You mix the butyrate dope and thinner 50/50 I know, but how much powder do you add for proper consistency?
Should the mixture be like pancake batter, maple syrup, or a bit thinner? And how many coats?

RE: Plasticized dope mixture
 

It sounds like you are "mixing" processes. You would add castor oil to the thinned dope to plasticize it and help keep it from hardening too much and cracking.

You would add talcum powder to the thinned dope to make your own sanding sealer.

I don't recall doiing both to the same coat of dope. Plasticized dope would be used on silkspan. Sanding sealer would be used on uncovered wood.

But it has been probably 55 years since I did either, and i could be wrong.

Maybe contact Brodak, they seem to cater most to silk (span) and dope type finishes.

Good luck.

RE: Plasticized dope mixture
 

The talc dope mix should be pretty thin so that you can easily brush it on without the brush dragging. How many coats depends on how much you sand off after each coat. To me it is very important to sand most of it off. I don't remember the ratio of castor oil to dope is anymore.

jamesf

RE: Plasticized dope mixture
 

About 6 drops of Castor Oil to a cup of thinned (50%) dope to plasticise over silk or silkspan. The talc should only be used over solid wood or silk covered wood to fill the grain. You then sand most of it off untill you can see but not feel the grain or weave. I would not use dope and talc over silk or silkspan.
Regards

RE: Plasticized dope mixture
 

Maybe I have the term wrong but according to Jim Ryan's instructions . . .
Found here http://home.fuse.net/ryan/silkspan.htm

Quote step 8: "Mixing Filler: To fill the weave preparatory to painting, mix regular talcum powder into your thinned dope until you have a slurry. You'll have to acquire a feel for how much to use; too little and it doesn't fill very well and is harder to sand; too much, and the filler will be porous and won't take a smooth paint surface. The right mixture has the consistency of watery wallpaper paste. "

As I don't know what a slurry is I don't know how thick that is. And I have never worked with wallpaper paste so I have no idea how thick that is.

I am concerned as I dont want to use too little and not be able to sand, and I dont want a big mess from it being too thick. As I am using butyrate dope I need this mixture to stop the tautening of the silkspan, as well as stop any cracking.

So how thick is wallpaper paste and a slurry?

RE: Plasticized dope mixture
 

the talc and dope should be like cream, not thick like whipped craem but like cream straight out of the bottle. I would never use sanding sealer (Dope and Talc) over any open frame covered surface. If you do, then when you are sanding you will surely cut through the covering at the ribs. Also it is not necessary, just lay on enough plasitcised dope to get a shiny finish. Lay it on with the brush held almost parallel to the surface

Regards

RE: Plasticized dope mixture
 

As I do not want to buy castor oil just for 5-6 drops I am going to use simply 50/50 dope/thinner mixture and then paint with 50/50 thinned colored butyrate dope, and hope for no cracking. The entire plane is solid balsa except for the stab which I needed to cut out lightening holes.

RE: Plasticized dope mixture
 

If the model is all balsa, then you probably wont need any plasticiser. It is mostly usefull for open frame covering. You can also use the plasticiser used by the auto body shops when painting plastic bumpers. (probably more expensive than castor).

Regards

RE: Plasticized dope mixture
 

Thank you all for this info. So you are saying it will be ok with simply dope and thinner?
I was a bit worried as they say that butyrate dope continues to tighten long after the model is completed, which can cause warps.

RE: Plasticized dope mixture
 

I would not bother with plasticiser unless I was doping an open frame structure. the balsa will be solid enough not to flex enough to cause the dope to crack (in my opinion).
The silkspan will fill most of the grain and you will not need too much dope. The talc and dope does fill the grain and weave well. Be careful sanding the edges not to cut through the silkspan. I mainly use pure silk and although I don't get a super shiny finish, I use coloured silk and finish up with a nice "sheen" that looks good on an old timer.

Regards

RE: Plasticized dope mixture
 

Modern dope does not need the plasticizers; they already have them. There was a small revolution, chemically, when Dacron was first used in the 50's to cover real airplanes over cotton; they had to change the formulas to make it more pliable. The EPA made them change it again in the 90s, so now it is even softer than it use to be as a byproduct.

RE: Plasticized dope mixture
 

The 2 oz bottles at the Pharmacy are affordable and High grade as intended for internal use. Some, myself included have overlooked this convenient size and not equated the cross applications. It certainly adds longevity to a finish if your model ever lives so long. I never noticed it added to the shine. 4 to 5 running drops per quart of dope. All fish eye reducers, flex agents for painting bumpers etc. all go back to the bean. Any left overs may be added to your fuel depending on your engines requirements. It also plays a part in not allowing the butrate to shrink quite so much over time which also adds to finish life. In the old days of magazines that had the craftsman type modeler in mind they could devote a couple pages to why it should be used. I use a Tom Brown Product called Flex-all. It has a handy container for running drops of what I presume is just the same old bean oil. One container lasts a long time.

RE: Plasticized dope mixture
 

Non taughtening nitrate and butyrate dopes have been plasticized by the manufacturer. I use them on silk covering and nary a warp develops. Use the appropriate thinners for each, clean your brushes and spray gun with acetone or mek.

RE: Plasticized dope mixture
 

Rather than sanding silk, or silkspan, over open bays, I use steel wool. Just have to run a magnet over it when finished, which will gather an amazing amount of steel fuzzies.

RE: Plasticized dope mixture
 

I have found Scotchbrite to be better for me than steelwool. The Scotchbrite (SB) leaves no metal bits to rust and is available in different "grits" at auto refinishing materials shops. The SB is identified by color. The green variety available at grocery stores works fine for me for most uses, as well as those in the kitchen when I do the pots and pans. I think the SB is an abraisive grit in a nylon nonwoven material.
Good luck

Sincerely,

Richard

RE: Plasticized dope mixture
 

I agree. The finish on my Pitts M12 (my avatar) is butyrate dope over nitrate over dacron glider fabric. It's still as bright and flexible as when it first flew over five years ago with no signs of cracking or aging. I just finished a minor repair to one wingtip that I accidently bashed into the shop door and the repair is almost invisible with the new colors matching the old perfectly. Modern butyrate needs nothing more than thinner, retarder, and patience.

Curt