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Building Tips and technical articles. => Paint and finishing => Topic started by: Jim Oliver on September 23, 2010, 07:50:30 PM
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I have recently had a problem with Sig clear Lite Coat crazing when applied over Sig Super Flite colored silk span and Sig colored Lite Flite tissue. I have used only Sig dope and thinner and have brushed it on in my air- conditioned shop. I haven't noticed the problem when used over white (uncolored) silk span....
I thought at one time that it might be something goofy going on with the Sig Nitrate clear ao I stopped using the Nitrate. It's still happening with Lite Coat. I am trying Super Coat clear on a test piece to see if it's any better.
It seems to make no difference how long I let the stuff dry before brushing on another coat.
The crazing can be sanded out but comes back with the next coat........
It seems to show up only after several coats of thinned clear have been brushed on. HB~>
Thanks for any help---or sympathy, even!!
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Ty,
Different brush.........
Jim
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Crazing is caused by contamination. Every time. Something incompatible between the dyed paper and dope or perhaps the paper was contaminated in some way. But crazing is caused by contamination. What is the paper dyed with?
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Randy,
My thought, also. The "paper" is Sig Lite-Flite (Jap-like tissue) dyed at the factory?? Also, the heavier Sig Super Flite (sorta like med. silk span) dyed at the factory......
Another clue, no problems with the same dope/thinner/air temps/brush/position of tongue in mouth when using undyed silkspan.......
I really like working with the dyed stuff, but looks like it's back to square one.
One sorta "funny" thing about this, there seems to be no "crazing" of the clear where the colored paper is applied over sheeted surfaces------and, I have seen no crazing when colored dope is applied. Sure has me baffled.....
Thanks,
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In purly paint terms, crazing is normally what happens when you spray laquer over enamel, the result looks like the surface just has cracks in it. This happens when the substrate absorbs thinners, and swells slightly, the top coats dry, then the solvent finally evaporates from the undercoats shrinking and pulling the dry top surface apart elaving minature grand canyons. You can fill them but the same problem will continue to occur,,
My guess is that the dye is a problem, or that there is some sort of sizing on the paper, though if the nitrate skims off, leaving solvents in the undercoats, when you hit it with lite coat, it will not allow the solvents to penetrate the skim layer on the nitrate and will cause swelling and crazing,,
what thinners are you using in your material?
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Using all Sig stuff, dope and thinner-----from more than one can. Also, have not used nitrate at all on the last job.
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I have had this same problem before using nitrate and then butyrate over it. Even on white silkspan. I quit using nitrate all together. Jim Lee also had this problem a few years ago I believe, on a plane he had at VSC. Same thing, butyrate over nitrate. I still have a Super Zilch that looks kind of mottled. Some people get away with it when using color dope over it, but for clear finishes it has never worked for me.
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I have had this same problem before using nitrate and then butyrate over it. Even on white silkspan. I quit using nitrate all together. Jim Lee also had this problem a few years ago I believe, on a plane he had at VSC. Same thing, butyrate over nitrate. I still have a Super Zilch that looks kind of mottled. Some people get away with it when using color dope over it, but for clear finishes it has never worked for me.
Hi Jim,
I would expect that the problem of the butyrate over the nitrate was caused as Mark said. The new layer hits the old layer (which has not gassed off) too soon and traps solvents which gas off at a different rate. In my case, this has never happened, but there is usually an extended time (laziness) between the nitrate and any thing else over it.
Bill
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I've re-covered 3 full-size planes using Ceconite fabric, nitrate first coats, then butyrate clear. Finally, automotive acryllic colors over all that! Sounds like a recipe for tragedy, but it worked perfectly in all cases. In fact, the instructions for Ceconite (unshrunk Dacron ) call for that very sequence. So if it works on all those planes, it should be no problem on models.
p.s. all dopes and thinners were Randolph from Aircraft Spruce & Specialty.
Floyd
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I am currently experiencing this problem with a Ringmaster covered with Polyspan. I started spraying with a thicker mixture of Butyrate over the Nitrate, That seemed to solve the problem. After several coats of the 75/25, dope to thinner, I went back to the 50/50 mixture and the crazing has returned. By the way, I let the Nitrate gas off for at least a week. Apparently, the thinner is loosening the substrate and the Nitrate is crazing the Butyrate. All products were Sig.
Jim
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I think Ty hit the nail on the head with the solvent...it is still gassing off as he said... the silkspan may be holding it longer and when you seal it with the next coat.. it has no where to go except through the next coat some how..in your case by shrinking the top coat... some old body guys always swore lacquer takes forever to dry because it has so much solvent in it plus what you thin it with... dope is a relative of lacquer... My guess would to cut back on the thinner some and give it plenty of time to dry.. I found out years ago.. never mix nitrate and butyrate.. (sp)... for what its worth...