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| Cat-urethane question http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=25310 |
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| Author: | old man [ Mon Jan 04, 2010 10:12 pm ] |
| Post subject: | Cat-urethane question |
My first 9 were all done with a tru-oil finish, and I like that. But I'm going to let my brother (Wisdom Paint & Body) put on a cat-urethane finish on one for me. My question is how many coats are necessary? I assume, on the top, to keep it as thin as possible, just apply what is necessary to make it smooth and even. Experienced thoughts? Ron |
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| Author: | old man [ Mon Jan 04, 2010 10:47 pm ] |
| Post subject: | Re: Cat-urethane question |
Thanks, Todd, I was hoping you would answer. Ron |
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| Author: | woody b [ Tue Jan 05, 2010 7:01 am ] |
| Post subject: | Re: Cat-urethane question |
It depends on how thick the wet coats are and how much it shrinks. Urethane doesn't shrink as much as lacquer but it still shrinks some. I'd recommend spraying some scrap, and then measuring the thickness. You're looking for a final film thickness of 4 to 5 mils. |
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| Author: | Tim McKnight [ Tue Jan 05, 2010 12:05 pm ] |
| Post subject: | Re: Cat-urethane question |
Here is my schedule: Pore fill and wait 2 - 3 days. Shoot 3 [wet] coats at 1.5 hour intervals. Wait 48 hours and level sand. Shoot 3 more on sides and back, two on the top. Wait 14 days and buff. You can buff as soon as 3 days but you can have shrink back. If I wait 14 then I never have any shrinkage issues. |
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| Author: | stan thomison [ Tue Jan 05, 2010 1:14 pm ] |
| Post subject: | Re: Cat-urethane question |
Ron where do you get tru oil now? I went to Walmart where used to get it, but since they quit selling guns they don't carry it anymore. Also went to Bass Pro here in Branson, and they don't carry it either, here or Springfield. |
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| Author: | Steve Davis [ Tue Jan 05, 2010 2:24 pm ] |
| Post subject: | Re: Cat-urethane question |
You're looking for a final film thickness of 4 to 5 mils.[quote][/quote] Isnt that super thick? |
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| Author: | Darrin D Oilar [ Tue Jan 05, 2010 3:01 pm ] |
| Post subject: | Re: Cat-urethane question |
I believe a mil is .001"...not millimeters. Darrin |
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| Author: | ChuckB [ Tue Jan 05, 2010 3:03 pm ] |
| Post subject: | Re: Cat-urethane question |
Todd, do you spray the nason 496 over shellac? And, are auto paint suppliers the best place to buy the nason? Thanks Chuck |
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| Author: | Robbie O'Brien [ Tue Jan 05, 2010 5:36 pm ] |
| Post subject: | Re: Cat-urethane question |
Ron, I usually put on 5-6 coats when using cat urethane. Remember it is not how much you put on but how much you leave on. Good luck. |
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| Author: | woody b [ Tue Jan 05, 2010 6:10 pm ] |
| Post subject: | Re: Cat-urethane question |
Robbie O'Brien wrote: Ron, I usually put on 5-6 coats when using cat urethane. Remember it is not how much you put on but how much you leave on. Good luck. That's true, but alot of catalyzed finishes don't burn in, or at least don't burn in as good as nitro. At least for me it's more important to do a good job spraying, so there isn't much leveling, or hard buffing necessary. You can get witness lines if you sand through one coat into the one under it. A "mil" or at least the mil I'm talking about is .001", or 1 thousant of an inch. The metric measurement is "microns" 25.4 microns make 1 mil. 1000 microns make 1 millimeter. A final finish thickness of 4 to 5 mils, or 100 to 125 microns is what I'm looking for. Todd made a really good point about solids content, wet film thickness, and shrinkage. I had never thought of it that way. |
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| Author: | old man [ Tue Jan 05, 2010 10:36 pm ] |
| Post subject: | Re: Cat-urethane question |
Many thanks for the info guys, I'll print this thread and give it to my brother. Ron HEY, STAN, I've been ordering direct from Birchwood Casey. http://www.birchwoodcasey.com/sport/index.html |
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| Author: | old man [ Tue Jan 05, 2010 10:42 pm ] |
| Post subject: | Re: Cat-urethane question |
Hey, Todd (and others), since the top doesn't need filling, would you spray directly on the spruce or still seal with a coat of shellac first? Ron |
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| Author: | Robbie O'Brien [ Tue Jan 05, 2010 10:46 pm ] |
| Post subject: | Re: Cat-urethane question |
old man wrote: Hey, Todd (and others), since the top doesn't need filling, would you spray directly on the spruce or still seal with a coat of shellac first? Ron You can spray direct if you want. |
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| Author: | Robbie O'Brien [ Tue Jan 05, 2010 10:48 pm ] |
| Post subject: | Re: Cat-urethane question |
old man wrote: Hey, Todd (and others), since the top doesn't need filling, would you spray directly on the spruce or still seal with a coat of shellac first? Ron You can spray direct if you want. |
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| Author: | Daniel Minard [ Tue Jan 05, 2010 11:15 pm ] |
| Post subject: | Re: Cat-urethane question |
Is this Nason finish different than catylized polyurethane? I used Becker Acroma polyurethane system on several instruments & really liked it. According to the tech rep I spoke to, this stuff is formulated for wood finishing, not the automotive industry. A dream to spray, levelled really fast, buffed easily to a beautiful shine... All the good stuff. I didn't leave it for three weeks before final levelling, so I did get some shrinkback into the pores. Also, on the down side, on Bigleaf maple, the finish looks quite milky. It looks fine in artificial light, but outside on a bright day, it is quite noticeable. I used polyester on my last several instruments & had some serious difficulties with it. If I could find a polyurethane which works well on maple, I'd be happy to give it a go! Have any of you experienced guys found the milky effect with the Nason finish? Now I just have to find out if I can get it in Canada... Good thread. Thanks for getting this ball rolling Ron! Oh yeah... to mask the soundhole, I stuff a piece of 4" mattress foam into the soundhole & sandwich a disc (1/2" larger than the hole) of cardstock (Bristol Board) between the top & the foam. This makes a great seal & has the added advantage of a ready sample of the finish thickness. Just pop out the cardstock disc, measure the clean edge with a micrometer & compare with the thickness of the finished portion. Thanks again guys! Dan |
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| Author: | Tim McKnight [ Wed Jan 06, 2010 12:07 pm ] |
| Post subject: | Re: Cat-urethane question |
woody b wrote: That's true, but alot of catalyzed finishes don't burn in, or at least don't burn in as good as nitro. You can get witness lines if you sand through one coat into the one under it. Woody, McFaddens is what I shoot and it is formulated for wood, specifically musical instruments. It does burn in BUT only if you shoot successive coats under the 6 hour pot life. Otherwise if you wait longer than that it will only adhere to previous coats via a mechanical bond (sanding). I have never experienced any witness lines with this product except when trying to figure out how to spot repair early on. I am not familiar with automotive urethane's but I would advise NOT to shoot McFaddens cat urethane directly over epoxy or shellac because delamination will occur. You need to shoot a cat product over a cat vinyl sealer or a cat RW sealer and the sealers must use the same hardener as the topcoat to be compatible. The finish will appear to stay in place until you ding it and then the "white spot" will appear which is classic delamination. |
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