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Building Tips and technical articles. => Paint and finishing => Topic started by: Dennis Toth on July 19, 2012, 08:40:40 AM
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Guys,
I was wondering what is added to dope to make it non-tauting? I heard that GMA use to add castor oil but this seems like it would reduce the grip of the paint to the surface.
Second question - after we get the covering adhered to the frame why do we brush the first few coats on instead of spraying?
Best, DennisT
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Don't know the answer to your first question.
I brush the first coats of clear, too lazy to hook up spray and clean it afterwards.
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I don't know what manufacturers do to reduce taunting. I use both taunting and non-taunting dope in the process. I usually don't spray until primer goes on, but that is more about wanting to use unthinned dope. It doesn't spray very well.
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I use butyrate dope all te way thru now after a little fiasco of getting nitrate and butyrate mixed up. If you try spraying the thinned dope for the first coats to seal the material used you will learn real fast why we brush the first coats to seal the material. I tried it many, many years ago when I got my first spray kit. Did not take long to realize I was blowing the dope right on thru the material. H^^
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I brush all the dope up until everything is sealed then I get out the spray guns.
Castor oil is the ancient method of "plasticising" dope. I use a PPG product that goes in the paint that is used on the "rubber noses" of modern cars so it won't crack. Wasn't expensive when i got it, but that has been years ago now......
I still use Nitrate (Ranolph's tinted, non tautening) until I have every thing covered, then switch to butyrate. Tautening for open bays (3-4 coats, thinned) and then non tautening for the rest.
BIG Bear
RNMM/AMM
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Guys,
I was wondering what is added to dope to make it non-tauting? I heard that GMA use to add castor oil but this seems like it would reduce the grip of the paint to the surface.
Second question - after we get the covering adhered to the frame why do we brush the first few coats on instead of spraying?
Best, DennisT
Hi Dennis,
When finishing models I use dope thinned as required out of the can. The reason for brushing on the first three or so coats is that the brush will act as a squeegee pushing the dope through the silkspan or silk to fully encapsulate the covering material (on both open-bay areas & on balsa), providing maximum grip of the dope to the substrate.
For the first three coats on the open bay areas only I use tautening dope thinned as required with no plasticizer added, brushing it on to tauten the covering. When about 3 coats of tautening & non-tautening dope have been brushed on (non-tautening for the solid areas & tautening for the open bay areas), for all further dope through color I start adding a small amount (about 1/4 to 1/3 of the recommended) of Dave Brown's "Flex-All" plasticizer to prevent pulling of dope in the fillet areas for all of the dope finish through all of the color. For all of the final clear dope finish on top of the color I add the full amount of plasticizer recommended (& no more) to non-tautening dope to further prevent dope pulling up in the fillet areas. I highly recommend using Dave Brown's "Flex-All" plasticizer because it is a known quantity; when you start experimenting with castor oil, etc. you become an experimental chemist on your labor of love (whether it be a classic or current stunt weapon) & risk having problems that are easily avoidable.
And Dennis - you are correct in that plasticizer does reduce the inter-coat adhesion of the dope coats, so it is important to not overdo the plasticizer. There is a fairly fine balance between enough plasticizer to prevent pulling the dope in the fillet areas & too much plasticizer which will result in loss of inter-coat adhesion.
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I brush on the initial coats because it fills and builds the substrate faster than spraying.
Also you dont have to thin quite as much for brushing which saves on thinner.