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inverted flyer 02-21-2004 08:39 AM

dope question
 
Hello folks

I need some advice on dope ....I have some sig dopes but I'm told that I should add some oil to prevent the dope from cracking or 'crazing' when dry.
Is this true? If so, what exactly should I use and just how much?
Thanks in advance
I.F.

LesUyeda 02-21-2004 10:01 AM

RE: dope question
 
Been using dope since the early 50's and never added anything; still using it and never had any problems.

Les

linclogs 02-21-2004 10:06 AM

RE: dope question
 
What you're refering to is called "plasticizing". Years ago, we used to add a few drops of castor oil to the dope to do this. Not sure, but I don't think you need to do that with today's dope. I never do and my doped finishes last for years. I think the oldest model I still have is like around 20 years old and I don't have any problems with the finsh (and I use Sig brand). Usually a doped finish will far outlast the life expectancy of the model (at least they have on mine).

FHHuber 02-21-2004 05:25 PM

RE: dope question
 
The adding oil was generally used for turning dope into a micro-film ocvering for super-light indoor models. There were all kinds of interresting formulae on how to alter dope for this purpose. All included some oil, I know of one that involved 10% beer, 89% dope, and the other 1% was oil and other stuff...

Butyrate Dope hase less than half the shrinkage potential of Nitrate Dope. So generally for painting the wood ofr fabric covering, you don't add a plasticiser to buterate dope.

If Dope is being applied over some other paint... yes you can have severe crazing of the finish as the dope ages. You won't fully prevent that no matter what you do. The laquer base of Dope just does not fully bond to other paints. (not even to automotive type laquers.)

inverted flyer 02-21-2004 06:04 PM

RE: dope question
 
Thanks for those answers,

any tips on airbrushing
and what's the best way to seal an area where you dont want the dope to go ie. if you have a two-tone colour scheme with a thin silver stripe separating them?

I.F.

FHHuber 02-22-2004 12:39 AM

RE: dope question
 
Airbrushing:

ALWAYS have good ventilation when spraying Dope. Its mean stuff if you inhale the fumes. Its meaner to inhale the mist. This stuff can kill you if you spray in an enclosed space. It is also an explosion hazard in confined spaces.

Thin with either the SAME brand thinneras the dope... or Acetone. don't mix brands of dope... they sometimes congeal to the consitency of cottage cheeze when mixing different brands. (one heck of a mess...)

1) the dope MUST be thinned to airbrush. Usually 50-50. Thicker dopes may need MORE thinner. (Pactra Fokker Red needs about 40% thinner, 60% dope... its already thin. The thinnest color I have ever found, and its consistent.)
2) Mask off well using automotive "Fine Line" masking tape for the edge of the area to be sprayed and blue masking tape over the rest. If you don't want the new color on it... cover it.
3) one LIGHT coat of clear is sprayed at the edges to seal the tape to the old color. (clear may bleed... but you will never see it.)
4) spray the new color.

kdheath 02-22-2004 02:01 PM

RE: dope question
 
Dave Brown still sells Flex-All plasticizer. Or the above-mentioned castor oil still works. I have come to the conclusion that current dopes like Brodak (made by Randolph, that makes a lot of the dope used on full-size cloth covered planes) really don't need plasticizing. Sig seems to hold up well, too. AeroGloss has been changed so many times that it is hard to be sure what you are using-depending on the age of the bottle. Brodak is a sure bet though. Great stuff!


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