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Gold Top help
http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10123&t=43388		 Page 1 of 2
Author:  absrec [ Tue May 06, 2014 8:38 pm ]
Post subject:  Gold Top help
Hoping someone here is good at this finish.

I've sprayed this finish twice now and it's still not right. The first time, the surface prep wasn't up to snuff. Both times, however, I got a bunch of bumps in the finish after spraying the gold coat. I'm using the crescent bronze #256 mixed into lacquer.

From an airplane, the thing doesn't look half bad.
Image
But up close, I'm getting these little bumps.
Image
I feel like it's premature flashing. I tried spraying a wetter coat and it didn't really help much. The upper part of the body looks perfect. Could there still be a problem with my surface prep? This color seems incredibly unforgiving.


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Author:  Chris Pile [ Tue May 06, 2014 10:53 pm ]
Post subject:  Re: Gold Top help
Are you keeping the cup of liquid swirled around as you shoot?
Looks like the heavy stuff sank to the bottom as you worked your way down.

Maybe need to filter it before shooting?
Author:  B. Howard [ Wed May 07, 2014 7:52 am ]
Post subject:  Re: Gold Top help
Could be solvent pop. Solvent pops in metallics push the flakes around as they appear and this would fit with your feeling of premature flashing. A wetter coat likely made matters worse. The answer for this may have been some thinner or retarder. It could also be that you simply put down your coats to heavy. Mettalics like to be sprayed thin and sort of dry. My average wet film on a mettallic is only around 1-2 mils. I usually figure on 4-5 coats to get good holdout and even sparkle.

Surface prep is also very important, ANY small surface defect will be highlighted by the mettalic. I sand my sealer out to 600 grit before the metallic goes down. It must be sanded absolutely level, as if you were going to buff it almost. Repairs, filler and putty have a habit of ghosting through mettalics on wood when the clearcoats are applied due to the re-wetting of all the film from solvent bounce when spraying the clearcoats. metallic finishes are more challenging than bursts or tints.
Author:  absrec [ Thu May 08, 2014 12:11 pm ]
Post subject:  Re: Gold Top help
I gave it another shot. I thinned out the mixture a bit. It was just a tad on the high side. Not too bad but I wanted to rule that out. I also strained the paint twice and mixed well right before spraying.

At first it seemed like it was the one. However, this bumpiness seems to happen on subsequent coats. The more coats I do, the worse it gets. Brian, What pressure do you use when doing one of these finishes? My gun is set for around 30psi.


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Author:  absrec [ Thu May 08, 2014 9:45 pm ]
Post subject:  Gold Top help
I spoke with someone at Crescent Bronze corp. and they said that their #256 is suitable for all types of lacquer although it can get a little finicky with nitro. He recommended 1-1.5 pounds per gallon of lacquer. I think I was in that ball park before but since I've tried 3 times so far, I was almost out and needed to mix up a new batch anyway. I used a scale and think I got a good batch mixed. To compound matters, I think the vinyl sealer I used to seal the top has gone bad. It won't powder up. I was able to get back to it and resand with 600. I sprayed one light coat where I could still see a faint hint of grain behind it. I could see that I apparently didn't get all of the sanding scratches out from the previous stripping of the gold. This kinda sucks. And the guitar isn't even for me. Oh well...


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Author:  Chris Pile [ Thu May 08, 2014 10:58 pm ]
Post subject:  Re: Gold Top help
Practice makes perfect.
Author:  B. Howard [ Fri May 09, 2014 7:53 am ]
Post subject:  Re: Gold Top help
I have the regulator attached to the gun set to run 30 psi when the trigger is pulled. My gun also has an air choke which allows me to then adjust the volume of air rather than the pressure for fine tuning, this is done by eye and feel. What size needle and tip are you using?

Most sanding sealers undergo a type of conversion when the first coat of lacquer is applied and will harden and loose their easy sanding properties. This is by design and why it is always a good idea to stay within a system of finishes. As you can see, metallics are most unforgiving when it comes to scratches and defects in the sealer.

Does the problem always happen in the same area on the guitar like in the pics? Or does it happen at random all over the surface? What happens if you sand it back a bit?
Author:  absrec [ Fri May 09, 2014 6:15 pm ]
Post subject:  Re: Gold Top help
B. Howard wrote:
I have the regulator attached to the gun set to run 30 psi when the trigger is pulled. My gun also has an air choke which allows me to then adjust the volume of air rather than the pressure for fine tuning, this is done by eye and feel. What size needle and tip are you using?

Most sanding sealers undergo a type of conversion when the first coat of lacquer is applied and will harden and loose their easy sanding properties. This is by design and why it is always a good idea to stay within a system of finishes. As you can see, metallics are most unforgiving when it comes to scratches and defects in the sealer.

Does the problem always happen in the same area on the guitar like in the pics? Or does it happen at random all over the surface? What happens if you sand it back a bit?

I'm spraying around 30psi. Using a door jamb gun with a 1.8 needle. It tends to happen behind the bridge area. I did sand back more in that area this morning. I went as far as using a lighted magnification thing I started using for fretting and other detail work. I saw a few scratches and leveled them out. If there are any scratches left to show up under the next coat of gold I do, I give up. It looks clean as far as I can tell. It's raining today so I'll probably have to wait until next week before trying again.
Author:  B. Howard [ Sat May 10, 2014 8:09 am ]
Post subject:  Re: Gold Top help
Needle is big for this application which could cause some atomization and therefore flow out problems. I use a 1.4 but am also mainly using high solids coatings, a 1.2 may be best depending on your lacquer but 1.8 is big. Is there anything different about the area below the bridge as far as prep? Any filler in that area?
Author:  absrec [ Sat May 10, 2014 11:08 am ]
Post subject:  Re: Gold Top help
B. Howard wrote:
Needle is big for this application which could cause some atomization and therefore flow out problems. I use a 1.4 but am also mainly using high solids coatings, a 1.2 may be best depending on your lacquer but 1.8 is big. Is there anything different about the area below the bridge as far as prep? Any filler in that area?

I don't think I did anything different but who knows. Wood prep has never been my forte but I am quickly learning a lot about it and the importance of it as well.

Are you saying I'll never get it with the 1.8 or that I just need to make certain concessions?

I've been watching videos and reading up on spraying metallic finishes. Most of this info are people using rattle cans and everyone seems to be misting the finish on. Probably due to the low pressure and high volume of an aerosol can. When I'm spraying with a gun, I would guess my tip is about 6 inches from the surface. I've thought about increasing the flow and spraying from further back but this just seems like it would result in more loose particles and increase the potential for premature flashing.
Author:  Chris Pile [ Sat May 10, 2014 11:14 am ]
Post subject:  Re: Gold Top help
When I spray metallic finishes, I usually hold the gun a couple inches farther away from the work to avoid stripes. Doesn't seem to be your problem, however.

Since you are doing just the top (not the whole instrument), you might try laying it flat.... 'cause, if what you doing isn't working, try something else. Just sayin.
Author:  B. Howard [ Mon May 12, 2014 7:24 am ]
Post subject:  Re: Gold Top help
The large needle and tip size is not making it any easier but it should be possible. You will be forced to put the coats down wetter than I typically would which could be leading to some solvent pop. Try a bit of retarder perhaps instead of thinner. Welcome to the world of finishing.....sometimes that's the way it goes. Did a job in Inca silver a while back, looked great till I buffed it out and I found that the metallic particles shifted from solvent bounce over an old repair and lit it up like a neon sign......stripped and re-done on my dime and a customer that had to wait an extra month for his guitar.
Author:  Glen H [ Mon May 12, 2014 7:38 pm ]
Post subject:  Re: Gold Top help
Have you looked closely to make sure there is not a build-up of finish at the end of your spray tip? Long shot but thought I'd ask.


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Author:  absrec [ Wed May 14, 2014 7:31 am ]
Post subject:  Re: Gold Top help
Well... It ain't perfect. But then again, neither is Rock n Roll.

Image

What I found was -

1. I had too much thinner in the mix. There wasn't enough binder to keep the flakes from shifting around. Also, they were flashing before they ever hit the surface and overlapped stokes weren't burning in, rather they were building up in clumps.

2. Inevitably, I would have the ubiquitous hair or dust nib make it's way on there when doing full coats. I just drove on and when it was dry I lightly knocked those down with 400 and sprayed over them. Pretty seamless.

3. I don't know for sure why, but the dished out areas on the top were the main place the flashing/solvent pop was happening. Because if this, I figured I was spraying from too far away. I think I was right because when I moved closer everything seemed to lay correctly. The dished out areas were slightly further away from the tip of the gun and it must've given the lacquer just enough time to dry before hitting the surface and ruining my day. Very delicate balance between perfect and catastrophic.

Anyway, thanks to everyone for their help. I have one more of these to do and I hope I never have to do another one. :)


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Author:  B. Howard [ Wed May 14, 2014 8:00 am ]
Post subject:  Re: Gold Top help
absrec wrote:
I had too much thinner in the mix. There wasn't enough binder to keep the flakes from shifting around. Also, they were flashing before they ever hit the surface and overlapped stokes weren't burning in, rather they were building up in clumps.


The reason it sprayed better without thinner is that your needle and tip are large for this type material. You would need to add close to 50% reducer to thin the binder enough that it would start to be a problem. If it was flashing to fast with the reducer you were using that is an indication of the wrong speed reducer being used. Anyway glad you got it done. The next one will be easier ;)
Author:  Chris Pile [ Wed May 14, 2014 8:29 am ]
Post subject:  Re: Gold Top help
Looks great now!
Author:  dzsmith [ Wed May 14, 2014 12:00 pm ]
Post subject:  Re: Gold Top help
Looks great Aaron!
Author:  Glen H [ Wed May 14, 2014 8:02 pm ]
Post subject:  Re: Gold Top help
Sure does and I'm sure you have some satisfaction knowing that you persevered through it! Good job.0


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Author:  absrec [ Fri May 16, 2014 1:38 pm ]
Post subject:  Re: Gold Top help
I think I spoke too soon. After letting it cure for a couple days, I cleaned it off, cleaned my clear lacquer gun, etc. and proceeded to start with some clear coats. First, I hit it with a little heavier than a tack coat. After about 15 minutes I went over it with a full wet coat. It looked fine at first but then I saw this -

Image

I never saw this after spraying the color coats or the first coat of clear. Any idea what might have happened?


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Author:  Chris Pile [ Fri May 16, 2014 2:58 pm ]
Post subject:  Re: Gold Top help
What do you mean, "cleaned it off"? Did you scuff the metallic gold? Or just hit it with a tack cloth?
Author:  absrec [ Fri May 16, 2014 5:58 pm ]
Post subject:  Re: Gold Top help
I lightly wiped it with a paper towel damped with Naptha. It had been sitting for a couple days and had collected dust.


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Author:  Chris Pile [ Fri May 16, 2014 8:09 pm ]
Post subject:  Re: Gold Top help
Well, I would never use a paper towel for that (too much lint), but that is not causing your discoloration troubles. I'm stumped at this point.
Author:  Glen H [ Fri May 16, 2014 8:54 pm ]
Post subject:  Re: Gold Top help
Did you did use the same finish as before?


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Author:  absrec [ Sat May 17, 2014 7:11 am ]
Post subject:  Re: Gold Top help
I did. The only thing I can think of is maybe when I sanded certain spots and re-sprayed them I didn't get enough finish in those areas to match up with the rest of the body. I know it looks kind of black in the picture but it's really green kinda like when you let the finish sit in the jar and the flakes settle to the bottom.


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Author:  B. Howard [ Sat May 17, 2014 7:26 am ]
Post subject:  Re: Gold Top help
The metallic re-wetted and re-flowed when you hit it with the clear. The old guys I learned from called this "solvent bounce". What this means is that with every coat of material you put down the solvents in it actually re-dissolve the coating already down on the surface. This is how that lacquer chemically bonds or "burns in" as they say. If you add reducer to your clear this reaction can get worse. on most wood finishes this is not a problem but on a metallic job it can allow the flake to move and re-distribute causing problems like this. The only time I really have issues like this is over repairs, if the work is not tight or the wrong type of filler used it changes the "bounce point" where the solvent kind of reverses after a coat and starts drying. The difference in dry times causes disturbances in the flake pattern. I typically spray out a few light bond coats with zero reducers added over the metallics to reduce this phenomenon. This is part of why any finisher who is experienced charges the premium they do for metallic paint jobs.
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