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Why spray nitro in multiple sessions?
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Author:  James Orr [ Thu Feb 16, 2017 6:33 pm ]
Post subject:  Why spray nitro in multiple sessions?
I'm getting ready to shoot some Cardinal lacquer next week, and as I gave my notes a read-through to make sure I didn't forget the details, I started wondering why we shoot in multiple sessions over a period of a few days rather than simply putting in one longer day?

I imagine it has to do with giving each set of 3-5 coats more time to gas off before more finish gets applied over them, but is that correct? And is that necessary? It's become a custom, so I imagine it's a best practice, but I'm always curious about these things.
Author:  jshelton [ Thu Feb 16, 2017 6:52 pm ]
Post subject:  Re: Why spray nitro in multiple sessions?
James Orr wrote:
I'm getting ready to shoot some Cardinal lacquer next week, and as I gave my notes a read-through to make sure I didn't forget the details, I started wondering why we shoot in multiple sessions over a period of a few days rather than simply putting in one longer day?

I imagine it has to do with giving each set of 3-5 coats more time to gas off before more finish gets applied over them, but is that correct? And is that necessary? It's become a custom, so I imagine it's a best practice, but I'm always curious about these things.

I'll let someone with more chemistry knowledge give you the exact reasoning but I've found that many vary light (thin) coats with level sanding in between result in much less chance of crazing and a more durable and thinner finish.
Author:  StevenWheeler [ Thu Feb 16, 2017 7:32 pm ]
Post subject:  Re: Why spray nitro in multiple sessions?
I recall one builder recommending 9 coats in 1 day but don't remember all the details, perhaps it is/was C.F. Martin's spray schedule. Your prep and spraying has to be spot on for that to work. I think the 3 coats and level schedule came about as it gives you a lot more chances to repair the little things you missed.

Steve
Author:  Woodie G [ Thu Feb 16, 2017 7:47 pm ]
Post subject:  Re: Why spray nitro in multiple sessions?
I would suggest discussing this with the vendor - they will have the best insight into how their product handles in service based on application schedule variations. I am uncertain as to why a drying finish that fully burns into a unitary film would benefit from the sort of drying/sanding cycle needed for curing finishes like traditional phenolic/tung or urethane varnishes which retain their multi-layered structure after cure.

We used to apply half the top coats on Day 2 of the schedule, then wet sanded to 95% level, then final set of coats on Day 3. In comparing labor hours and drying time, the advantage to sanding between sets of coats - a reduction in the time to sand out and buff - is a wash with waiting until all coats are on.

I assume one could argue that a faster gas-out would occur with drying of the first set of coats, but that dry film would absorb solvent and other volatile organic compounds from the second set of coats in any case, and - we use a forced air drying box that speeds up drying to a noticeable extent over the usual 21 day wait - the slight variation in total drying time which might occur with a pause is not something we would have noticed.
Author:  Joe Beaver [ Thu Feb 16, 2017 10:17 pm ]
Post subject:  Re: Why spray nitro in multiple sessions?
I like Cardinal Luthierlac, but I have just done one guitar with it thus far.

The spray schedule greatly depends on the spray conditions. Hot and dry? You can push it with more coats per day. But I would never do more than 5 coats with at least 1-1/2 hour between. I spray each coat so it is just fully wet. Any less may end up orange peel, any more may not dry properly.
Author:  jfmckenna [ Thu Feb 16, 2017 10:53 pm ]
Post subject:  Re: Why spray nitro in multiple sessions?
As a geologist I can't help but think of what's called mud cracks.

Image
Author:  James Orr [ Thu Feb 16, 2017 10:56 pm ]
Post subject:  Re: Why spray nitro in multiple sessions?
Joe Beaver wrote:
I like Cardinal Luthierlac, but I have just done one guitar with it thus far.


Yep. This will be my second guitar with it. I had to spray the last one three times because of what turned out to be waiting too long between spraying the sealer and top coats. The finish delaminated the first two times through.

This tends to be the typical learning pattern for me. ;)

I followed LMI's schedule for it the third time and really liked the results.
Author:  James Orr [ Thu Feb 16, 2017 10:58 pm ]
Post subject:  Re: Why spray nitro in multiple sessions?
Woodie G wrote:
I am uncertain as to why a drying finish that fully burns into a unitary film would benefit from the sort of drying/sanding cycle . . .


Exactly! Still I don't plan to buck tradition on this one. It's definitely something to ponder though.
Author:  Joe Beaver [ Fri Feb 17, 2017 12:33 am ]
Post subject:  Re: Why spray nitro in multiple sessions?
I'm with you on spraying the top coat soon after the sealer. I think you posted a caution about it. That really helped. I decided to do a test panel using different wait times and found that it was best to spray as soon as it was fingerprint safe.
Author:  B. Howard [ Fri Feb 17, 2017 8:15 am ]
Post subject:  Re: Why spray nitro in multiple sessions?
I typically do not.....I spray it all on the same day and then let it gas out for 4 weeks. You will actually do less sanding and spraying this way and use slightly less material to boot! I have done the time studies on this......

The dangers with wet stacking air dry coatings, especially these days with the newer solvent blends, are mainly solvent pops from not letting enough dry time between coats and sags.
Author:  dberkowitz [ Fri Feb 17, 2017 9:08 am ]
Post subject:  Re: Why spray nitro in multiple sessions?
I think the reason has less to do with chemistry specifically than with practicality. Multiple sessions and leveling allow one to be assured that you've filled every pore and leveled them before spraying your last sessions. Lacquer is a wonderful material, but if you buff through it's another 4 weeks before you can try again. The extra few days on the front end is worth the time versus the problem of delay of another four weeks (and more importantly, the delay in moving product out of your shop and getting paid).

Additionally, and it's been borne out, that lacquer likes time to outgas. This is the curing process of solvent based, non-catalytic finishes -- the solvents are simply a medium in which a plasticized material is carried either for brushing or spraying. Initially, the bulk of the solvent evaporates and the material coalesces on the substrate forming a film. It takes time to get that last amount of solvent to evaporate. Many schedules include a day between sessions in which the film is cut with a light sanding -- not to level, but to relieve the surface tension as the film cures and allow the solvent to move out of the film more easily. These breaks in the schedule allow each of the sessions more time to cure before adding to them, and giving better stability to the lacquer that is simply there to fill the pores.
Author:  SteveSmith [ Fri Feb 17, 2017 9:29 am ]
Post subject:  Re: Why spray nitro in multiple sessions?
I think a lot of it has to do with experience and knowledge. Brian is a pro and knows exactly what he can get away with. For those of us with lesser skills a less aggressive approach seems more prudent.
Author:  dberkowitz [ Fri Feb 17, 2017 9:36 am ]
Post subject:  Re: Why spray nitro in multiple sessions?
SteveSmith wrote:
I think a lot of it has to do with experience and knowledge. Brian is a pro and knows exactly what he can get away with. For those of us with lesser skills a less aggressive approach seems more prudent.


No offense to Brian, but if the factories (and finishers like Adam Stark) thought they could get away with it, they would. I'm glad it works for him, but from my standpoint of having used any number of finishes over the years from McFaddens, any number of waterborne and conversion varnishes, it doesn't seem prudent to me. Builders as a rule work on a shoestring and can ill afford the delay of an order from a burn through.
Author:  Woodie G [ Fri Feb 17, 2017 10:10 am ]
Post subject:  Re: Why spray nitro in multiple sessions?
If I recall correctly, Mr. John Hall applies all of his coats in one day, with 45-60 minutes between, but that information may be a bit dated. You might drop him a note at the Blues Creek site - he seems to be very generous with his time and always interested in helping other builders and repair people.

In discussion with the others in the shop, I was reminded that we have seen significant differences in how the vinyl sealers from different vendors handle - the old McFaddens stock that we had could be overcoated within 45 minutes without sanding, but the Mohawk sealer appears to be more sensitive to actual drying state with regard to adhesion between sealer and top coats. I would again suggest a discussion with the Cardinal techs to determine just how much latitude is available on sealer recoat or overcoating time for that finishing system.

One issue which may suggest a split spraying schedule is the number of hours of suitable weather conditions when spraying out-of-doors. It is unusual in this area to see the window for outdoor spraying extend much beyond 5-6 hours total duration per day, so getting 7 to 8 coats on can be a real challenge. Trying to push the window to get a final coat on after sunset and when the RH has bumped up to the 70% range can mean a third day in any case, as we shoot a final thinned coat to address the blush we encouraged.
Author:  Ben-Had [ Fri Feb 17, 2017 1:02 pm ]
Post subject:  Re: Why spray nitro in multiple sessions?
Here is my spray schedule for Cardinal lacquer. I have gotten excellent results from it. This is after all prep work (filling,staining etc.) and sealing the guitar with two coats of vinyl sealer, Take it for what it is worth.

First stage

0 Spray coat of clear on the B/S (spray edges first [top & back], then spray B/S this puts extra
finish on the edges to help prevent sand thru on the edges) Note: DO NOT spray the top on this
first coat, wait 30-45 minutes between coats
0 Spray coat of clear on the Top and B/S
0 Spray coat of clear on the B/S (spray edges first [top & back)
0 Spray coat of clear on the Top and B/S
0 Spray coat of clear on the Top and B/S

Second stage

0 Level sand w/320 grit (palm sander 1st/sanding block to finish up), clean intermittently
w/microfiber cloth
0 Wipe everting down with Naptha
0 Spray 3 coats, one every 30-45 minutes allow to dry overnight.

Third stage

0 Level sand w/320 grit (palm sander/block), 3M Grey pad and then clean with Naptha
0 Mix one cup of Lacquer (2 parts lacquer to 1 part thinner)
0 Adjust gun flow by closing off and opening until a steady stream comes out. Note: when
filling the cup leave fluid level 1” below top
0 Spray 4 coats every 30-45 minutes allow 2-3 days cure time.

Fourth stage
0 Mix 5 oz. of thinned lacquer and ½ oz. lacquer retarder
0 Adjust gun flow by closing off and opening until a steady stream comes out. Note: when
filling the cup leave fluid level 1” below top
0 Spray one Flash coat
0 Allow 10-14 days dry time

Fifth stage

0 Wet Sand P1200, P1500, Trizac 3000
0 Buff and polish with Menzerna (Atol-6 & P175)
Author:  James Orr [ Fri Feb 17, 2017 5:33 pm ]
Post subject:  Re: Why spray nitro in multiple sessions?
Great discussion! Thanks, guys. It's cool to compare the different views.

Joe Beaver wrote:
I'm with you on spraying the top coat soon after the sealer. I think you posted a caution about it. That really helped. I decided to do a test panel using different wait times and found that it was best to spray as soon as it was fingerprint safe.


Cool! [:Y:] I'm glad that was helpful. When your finish looks like a peeling sunburn, it's no bueno!


Ben-Had wrote:
Here is my spray schedule for Cardinal lacquer...

Third stage

0 Level sand w/320 grit (palm sander/block), 3M Grey pad and then clean with Naptha
0 Mix one cup of Lacquer (2 parts lacquer to 1 part thinner)
0 Adjust gun flow by closing off and opening until a steady stream comes out. Note: when
filling the cup leave fluid level 1” below top ...

Fourth stage
0 Mix 5 oz. of thinned lacquer and ½ oz. lacquer retarder
0 Adjust gun flow by closing off and opening until a steady stream comes out. Note: when
filling the cup leave fluid level 1” below top ...


Thanks for sharing that, Tim. Very helpful. I'm curious. You mention the way you set the gun and also thin the lacquer in your third, fourth, and fifth stages. Do you use the Cardinal un-thinned before those stages?
Author:  Ben-Had [ Sat Feb 18, 2017 12:41 am ]
Post subject:  Re: Why spray nitro in multiple sessions?
James Orr wrote:
Thanks for sharing that, Tim. Very helpful. I'm curious. You mention the way you set the gun and also thin the lacquer in your third, fourth, and fifth stages. Do you use the Cardinal un-thinned before those stages?

Yes.
Author:  bluescreek [ Sun Feb 19, 2017 8:04 am ]
Post subject:  Re: Why spray nitro in multiple sessions?
Day 1 seal and fill
Day 2 Seal and apply coats to a .016 coverage
let off gas
2 to 4 weeks
level sand
buff and polish

Martin used this same procedure but now with a computerized spray booth and air exchange room they are getting to sand in days not weeks
but then I don't have a few Million to spare for that spray booth.
Author:  Clay S. [ Sun Feb 19, 2017 8:55 am ]
Post subject:  Re: Why spray nitro in multiple sessions?
" I don't have a few Million to spare for that spray booth."

For some of us that is a good reason to finish over several sessions. Being able to nib sand between sessions because of a less than perfect setup can be a plus.
Author:  bluescreek [ Sun Feb 19, 2017 10:15 am ]
Post subject:  Re: Why spray nitro in multiple sessions?
talking about mixes I use Behlen and Mowhawk
I have 2 mixes
when I shoot my first 2 coats over the sealer I use 30% thinner and 20% retarder
after than a 10 and 10 mix
learning how to adjust the gun is key
I also have a separator on the air line to keep water and oil from contaminating the air. I have 2 guns that I use
1 is also known as a touch up gun
smaller than the qt cup HVLP
I like it as it is light and if I am doing 1 or 2 guitars it was my gun of choice.
With me getting so busy I now contract out my finish work to a retired Martin finish guy.
Author:  philosofriend [ Mon Feb 20, 2017 9:54 am ]
Post subject:  Re: Why spray nitro in multiple sessions?
In the bad old days a standard nitrocellulose lacquer technique for furniture factories was a set up for heating the finish before it got to the spray gun. The hot finish would flow out with much less solvent than cold spraying would have needed. This saved the cost of solvent and allowed a thick finish to be applied in one coat.
For those of us finishing normally, many thin coats can be sanded much quicker. Also, thin coats make smaller problems and can be repaired faster. These reasons matter less to someone who sprays routinely and has more skill.
Many thin coats help shorten the time until the guitar stops smelling bad.
Author:  Woodie G [ Mon Feb 20, 2017 11:30 am ]
Post subject:  Re: Why spray nitro in multiple sessions?
You may wish to review Cardinal's application video series for their musical instrument products; to summarize, they suggest that lacquer be thinned no more than 10% for a 1-1.5 mill dry film thickness per coat, and a two session (two day) build to final desired dry film thickness. Given most of us have to work with what we have and can afford in terms of application system and spray environment, further thinning or retarding may be necessary to address less capable spray systems and adverse conditions in the environment (e.g., hot and excessively humid or very dry, etc.).

On thinning and retarding - I have been taught that both sealer and finish should be thinned ONLY as needed to permit reliable atomization of the material in the equipment used and for the conditions in which spraying, with the goal of a wet coat of sufficient thickness to produce the desired dry film thickness after flashoff and drying, while avoiding excessive thickness which can produce solvent pops, film delamination (separation of the already partially dried lacquer film over which the coat is applied), or other issues. Similarly, we add retarder only when the increased drying time and reduced surface hardness is an acceptable exchange for being able to extend the range of spray conditions or address adhesion issues on aged or precat lacquer seen on some repairs. Thinning or retarding beyond that needed for successful application would seem to me to increase the number of opportunities for equipment, material, application, and environmental errors (bugs! pollen!), as well adding cost to the process in terms of materials and time.

Here is the link to Cardinal's application video; there are filler, sealer and final sand/buff videos as well.

https://www.youtube.com/watch?v=S2vIUNj7MR0
Author:  Ben-Had [ Mon Feb 20, 2017 11:55 am ]
Post subject:  Re: Why spray nitro in multiple sessions?
Woodie G wrote:
Here is the link to Cardinal's application video; there are filler, sealer and final sand/buff videos as well.

https://www.youtube.com/watch?v=S2vIUNj7MR0

I have been using Cardinal for at least 3 years now and tried their recommended method from the videos and it did not really work as well for me but as I said take my method for what it is worth. Equipment and spraying conditions can certainly vary and testing is probably a good idea.
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