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Author Topic: Pitting in Dope Finish  (Read 802 times)

Offline Phill Atkins

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Pitting in Dope Finish
« on: January 15, 2021, 11:01:20 PM »
I'm working on a dope finish over silkspan and I've done two primer coats and then two silver.  When I sand off the silver, I have pitting left behind.  I guess I probably hit the tissue, but what gets me is that the silver isn't filling it in.  I'll sand off the silver completely and there won't be any in the pit, as if I took a chunk of the dope off while sanding.  This last coat had 48 hours or so to gas off and seemed to sand ok.


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Online Craig Beswick

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Re: Pitting in Dope Finish
« Reply #1 on: January 16, 2021, 12:59:12 AM »
Is the silkspan on the right way up? How many coats on the silkspan before you sanded?

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Offline Phill Atkins

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Re: Pitting in Dope Finish
« Reply #2 on: January 16, 2021, 09:31:10 AM »
Is the silkspan on the right way up? How many coats on the silkspan before you sanded?

Craig
Well I brushed on a good five to six coats of clear butyrate at 60-40 thinner to dope.  As for right-side up, I've never heard of such a thing for silkspan.  Polyspan, sure.  But if there was a difference in the sides, I'd think I would at least luck out on one of the pieces I put on.  I did my wing in Polyspan and I do not have the pitting over the sheeting like I do with the silkspan.

Online Dan McEntee

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Re: Pitting in Dope Finish
« Reply #3 on: January 16, 2021, 11:34:39 AM »
Well I brushed on a good five to six coats of clear butyrate at 60-40 thinner to dope.  As for right-side up, I've never heard of such a thing for silkspan.  Polyspan, sure.  But if there was a difference in the sides, I'd think I would at least luck out on one of the pieces I put on.  I did my wing in Polyspan and I do not have the pitting over the sheeting like I do with the silkspan.

     As far as what you have, this is silk span over balsa sheeting or solid balsa?  If so, you may not have sanded the wood enough and with fine enough paper at the finish. Then the dope mixture might be a bit thin to fill the grain, which is what you want it to do. Then carefully sanding that until you get that flat, even if you have to recoat with some dope. Holding the part up to a light at an angle will help you see the low spots. Using a sanding black is also preferred.  I never heard about silk span having a right or wrong side. Japanese tissue, the good stuff, and the better domestic tissue, has a dull and shiny side, and you want the shiny side up. When you get the clear applications finished, then start with the primer and/or silver. If you are using silver, I wouldn't bother with the primer unless you know you can sand it all off easily, then you know the finish is flat, the apply the silver as a blocking or base coat.  You don't see anything with the Polyspan because it is a much tighter weave and takes a lot less dope to fill. On your plane, the dope is so thin is doesn't build up and the thinner just lets it soak in and sucks the silk span down into the spots where the grain was actually raised by the thinner. I hope that makes sense.
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Offline Phill Atkins

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Re: Pitting in Dope Finish
« Reply #4 on: January 16, 2021, 12:29:34 PM »
It's all 3/32" balsa sheets.  I finished the sanding at 320 grit, did two coats of thinned nitrate then two coats of thinned sanding sealer.  I've been thinking the same way as you I believe.  I feel like the wood wasn't fully sealed and the grain swelled, making small low spots where the primer filled.  I intend to use the silver as my undercoat, which is why I'm doing both.

I think I get what you mean though, the thinner is soaking in to the wood, raising the grain, and the tissue is pulled deeper into the new valleys leaving the pits.  What gets me is that the silver isn't filling in the pits when I spray (50-50 blend).  In other words, if I sand off the silver, I'd expect the pits to either still be silvered, having been untouched by the sanding, or sanded away.

I think what I'll do is spray the silver, wet sand it down until it starts thinning out, and spray the silver again.  Try to build it up without sanding it all away.  I can also check the viscosity of the dope I'm spraying to see if I'm reducing too much but I haven't had the impression that it's the case.

Offline John Leidle

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Re: Pitting in Dope Finish
« Reply #5 on: January 16, 2021, 08:00:04 PM »
 I seem to have the best outcome when I thin the paint or dope quite thin. touch sand when dry & build up slowly. I've had pits also ,, you name it I've had the trouble.

    When I hurry I have the most trouble.
     Best of luck to you , John L.

Offline Dave Royer

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Re: Pitting in Dope Finish
« Reply #6 on: January 16, 2021, 09:14:01 PM »
Those pits look exactly like the ones I got when I covered with nitrate and then went to butyrate. When I sanded through the butyrate into the nitrate I got pits that subsequently never completely went away. I think it's best to do all the filling and sanding with nitrate and then shoot the color and don't sand that or just do butyrate all the way from the wood up and play it safe.

Dave

Offline kevin king

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Re: Pitting in Dope Finish
« Reply #7 on: January 17, 2021, 12:06:13 AM »
Try adding more thinner. I thin my Silver to 30/70 and it sticks like snot on a screen door. I agree with Dave about leaving nitrate out of the finishing process. It could be caused by some type of contamination like water in the compressor line or not blowing off the part and cleaning it with m600. Temperature is important too. any cooler than 70f and the dope won't properly flow out, and the thinner wont evaporate, just sits there eating its way through the previous coats, silkspan, and into the wood. Is it an acrylic lacquer auto Primer? I try not to use any primer on my planes except for aluminum spinners and landing gear. People have successfully used dope over acrylic Lacquer, including myself but it's not compatible and too me not worth the risk.
   Kevin
« Last Edit: January 17, 2021, 12:53:58 AM by kevin king »

Offline Phill Atkins

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Re: Pitting in Dope Finish
« Reply #8 on: January 17, 2021, 12:51:06 AM »
It's all Brodak products, including the primer.  I'm still working on getting a good flat surface with the clear so it's been better for me to use the primer before the silver. Actually, I think the pits all are in the clear and not in the primer patches.  I haven't been able to source M600, but I'm using tack cloths and an automotive degreaser and surface prep cleaner (KW901 from Custom Shop).

Interesting about the nitrate and butyraye issues, I hadn't heard about those.  I was trying out the sanding sealer, which is butyrate based, so I only did two initial sealing coats in nitrate.  I've got a few more models next in the pipeline that I can try keeping all butyrate.

Online Dan McEntee

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Re: Pitting in Dope Finish
« Reply #9 on: January 17, 2021, 08:27:45 AM »
   You might try using all non-tautening products also, and whewn doing the initial filling, try some of Randy Smith's zinc sterate  powder mixed into the dope.
    Type at you later,
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Offline Phill Atkins

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Re: Pitting in Dope Finish
« Reply #10 on: January 23, 2021, 09:25:42 PM »
Figured I'd at least give a postscript to all this.  I went ahead and put down another dual coat of silver and wetsanded that with 1000 grit until the silver was starting to thin away.  This seemed to fix about 75% of the pits I had by not sanding away all the silver.  Then I did another dual coat of silver, wetsanded with 1000, and put on some insignia white.  Now that the white is gassing off, the surface is looking really good, at least until I dropped it.  So now I'll have to bondo the dent and sand and spray again.  Once I get the final cub yellow on I think it'll look just fine.

Hopefully Sig will get me my next order out soon enough and I'll be able to try again in 1-2 months on another plane.  I won't thin our the sanding sealer this time and probably still give an extra coat or two of clear beyond what I did with this one.  I'll probably still end up doing tautening butyrate because I only have a pint of Lite Coat and I do not want to add anything else to my standing order and have more delays.

Online Dan McEntee

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Re: Pitting in Dope Finish
« Reply #11 on: January 24, 2021, 04:28:22 PM »
   Hi Phill;

     While you are waiting on your SIG order, do some research into plasticizing the dope. I now there is a chemical for doing it but it is really bad juju as far as health concerns. Dave Brown Products has their FlexAll for plasticizing dope and I have used that in the past with good success. Back in the day, all the model finishing instructions called for add some castor oil to the dope for stick and tissue models and small c/l models. After using the Flexall, I think that it may just be castor oil, only more like medicinal grade, which is more clear. i actually tried tasting the two as a simple test and it tasted the same to me (just a small drop on my tongue.)  I think you only need to add about a table spoon to a quart of thinned dope. Trying this out in the early stages is a good time to work it out. I used the Flewall mostly in the colored dope I applied. If you are using SIG colors, it is blended from Litecoat as the base so you won't need it on the colors. Think about this and if you try it even on a test piece, report back what you find out to spread the information around.
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Offline Phill Atkins

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Re: Pitting in Dope Finish
« Reply #12 on: January 24, 2021, 05:43:34 PM »
I take it you mean that I can use the plasticizer to make my clear tautening butyrate behave like an un-tautening version, correct?  That's a good idea and I'll try to pick up a bottle of the Flexall.  I pretty much just get all my dope products from Brodak, Sig always is out of something that I have to wait weeks or months on and Brodak has a wider selection, but my understanding and experience is that their colored and crystal clear dopes are similarly non-tautening.  But it's nice to know I can still doctor up a batch from tautening colored dope too.

Online Dan McEntee

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Re: Pitting in Dope Finish
« Reply #13 on: January 25, 2021, 01:22:58 PM »
I take it you mean that I can use the plasticizer to make my clear tautening butyrate behave like an un-tautening version, correct?  That's a good idea and I'll try to pick up a bottle of the Flexall.  I pretty much just get all my dope products from Brodak, Sig always is out of something that I have to wait weeks or months on and Brodak has a wider selection, but my understanding and experience is that their colored and crystal clear dopes are similarly non-tautening.  But it's nice to know I can still doctor up a batch from tautening colored dope too.

     Yes, that is correct. If you can't find Flew-All, try some medicinal grade Castor oil from the drug store or you will find food grade 9which is basically the same thing) in the baking department of the grocery store. Do a little testing on amounts and see what you get.
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Offline Randy Powell

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Re: Pitting in Dope Finish
« Reply #14 on: January 26, 2021, 09:38:53 AM »
I use the Phil Granderson method.
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