Level sanding between coats of NITRO laquer, yes or no?

There seems to be conflicting ideas about this. Some say level sanding is required in between coats, but others state it's not required until the final coat because laquer melts into itself. There are many discussions about this on various guitar building forums, but I'm wondering what my friends here on UU think about this. Is it necessary ? If you think it is, can you please justify it with a logical explanation.
thanks Mike.
Please note, I'm talking about Nitrocellulose here and nothing else, so maybe some of you with extensive experience of using nitro might chip in here.

4 coats then level, 4 coats then level, 2 coats 50% dilution. Level, leave for 10 days then polish.

But if you want a girlfriend, remember that lacquer is NOT the same thing as shellac:

It's not necessary for adhesion. As you said in your post it melts into the previous coats. The only reason to sand is to keep things flat an take out any blips and unevenness. When using mat or semi gloss I level and spry two more coats and call it good.

Not sure what brand you are using but with Cardinal they suggest no more than two or three applied thinly per day with amply time in between. Ample depends on temp and if you are using retarder etc. They suggest some thinning and possible retarder as well. I just use the slowest thinner I can get and that seems to work pretty well. It drys to the touch quickly, it is tempting to keep shooting coats but doing so will greatly prolong hardening time.

Thanks. I'm sorry, I should have added that I use spray cans and high gloss. And Pete, I'm looking for reasons for having to do this, to help me understand. And Michael, you say the only reason to sand is to take out any blips and unevenness, which can be done after 12 coats then wetsanded to a finish. What's the difference? Or am I missing something here?
Cheers Mike.

With lacquer there is no benefit to adhesion by sanding. It's a chemical bond by melting into the previous coat. That is ONLY if you are spraying a medium to full wet coat. If you are dusting the coat on, you don't have enough solvent on the suface to soften / melt into the previous. This will cause you all kinds of problems, and I've seen people do this because they were afraid of getting runs.

Sanding is to take out the texture and imperfections that will ineveatbly be present after applying multiple coats. Depending on the enviromental conditions, your spray technique and the product / thinners used, you will want to go along with Pete's recommendations give or take a coat.

When I used lacquer it was 4-5 coats over my fully pore filled instrument. Minimum 20 minutes between coats. Often up to an hour. Let dry for 1 week. Level sand and repeat with 4-5 coats. Let dry for 2 weeks and then level sand out to P1200 and then buff to high gloss.

Pressure pack / spray bombs are the least desirable way to apply a finish for all kinds of reasons, but it can be done.

Allen said:

When I used lacquer it was 4-5 coats over my fully pore filled instrument. Minimum 20 minutes between coats. Often up to an hour. Let dry for 1 week. Level sand and repeat with 4-5 coats. Let dry for 2 weeks and then level sand out to P1200 and then buff to high gloss.

Pressure pack / spray bombs are the least desirable way to apply a finish for all kinds of reasons, but it can be done.

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Allen, there is no doubt in what you're saying. Your finishing speaks volumes and is a work of art. But would you not achieve the same finish by sanding out the imperfections at the end, sorry to labour the point but what would happen if you levelled after the final coat?
As for spray cans, it is my only option, being a hobby builder who completes 1 or 2 instruments a year, and not wanting a whole bunch of spraying equipment. I completed my first nitro build last year, which turned out reasonably well in the end after a lot of mistakes , and a steep learning curve. I want the next one to go a little easier.
Thanks for the reply. Mike.

No, you will not get the same finish without a level sanding between spraying sessions, and those sessions really need to be about a week apart when you are using lacquer.

I could go into a long explanation, but suffice it to say that if you want to get the best that you can from the process you are using, best to follow the recommendations. You will not get a great finish by leaving out any of the steps.

mikeyb2 said:

Thanks. I'm sorry, I should have added that I use spray cans and high gloss. And Pete, I'm looking for reasons for having to do this, to help me understand. And Michael, you say the only reason to sand is to take out any blips and unevenness, which can be done after 12 coats then wetsanded to a finish. What's the difference? Or am I missing something here?
Cheers Mike.

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Well theoretically yes you could go 12 coats then level in a perfect world, I do it all the time in my dreams. in a perfect spray booth with perfect prep and perfect application, but that world only exists in some factory settings. But they don't use nitro anymore anyway. Certainly not some guy with a couple rattle cans or small time shop for that matter. You can try it but you will find out that getting even a good high gloss lacquer finish is no cake walk.

Hey allen, that's really interesting. By letting the finish cure for two weeks half way through spray sessions, I'd imagine that you'd get a more fully cured finish, even if you used a four week cure post spraying. I'm getting ready to spray my first attempt at a high gloss finish and your method makes logical sense. Thanks

Lacquer never stops drying (if you want to think of it like that).

When you are spraying its easy to think that the finish is dry when it's dust free, but the volatile solvents in that first coat will not have fully gassed off by any stretch of the imagination before you are ready (say 20 minutes later) to apply another coat. It will have formed a skin that is dust free, and may even feel dry, but you would certainly still be able to smell solvent gassing off. A new coat also skins over and becomes dust free, but now you have heaps more solvent trapped in the film build. Reapeat to 4 or 5 coats and you get the idea.

These solvents will get out one way or the other. You don't want to bury them so deep in multiple coats that it would take 3 months instead of one to two weeks.

If you are good on the spray gun, it would be quite possible at the end of those 4-5 coats to see a finish that is pretty flat and you might be ready to pat yourself on the back. However in a day or so you will notice that surface is no longer as flat as you first thought it was. It sucks into the timber some, and you would enivatibly get some texture (orange peel) showing up as the solvents evaporate. I've been a proffesional spray painter for over 35 years, so I'm pretty damn good on the gun, and even I would see this.

This is why you level sand after a week.

And the same scenario goes with the next spray session. This one however would more than likely lay down flatter because you have a more sealed, level surface that just pore filled timber.

And as I said at the beginning, lacquer continuosly "dries". After the volatile solvents have gassed off the easiest way to think of it is that the finish starts to break down and degrade. Would depend on the formulation of the lacquer, but in round figures you have about 75 years before it's pretty much gone.

In the custom guitar world where surface finish thickness is hotly debated in the back room away from the public, the builder usually has a bit more thickness that would be ideal when it's new, because in 5 - 10 years it will be a fair bit thinner and potentially being the subject of a warranty claim.

Allen, Pete and Michael, thanks very much. I'm beginning to understand now after Allen's explanation. I hope I didn't come across as questioning your methods, far from it, but if there's something I can't quite grasp, I'm never too shy to ask. Building instruments is a fascinating pastime, with so many different skills and processes to learn. I don't just want to learn the processes, but also the reasons for doing them.
I'll go with your suggestion Allen and give a weeks wait in between sessions, then level etc.
Cheers Mike.

Michael Smith said:

It's not necessary for adhesion. As you said in your post it melts into the previous coats. The only reason to sand is to keep things flat an take out any blips and unevenness. When using mat or semi gloss I level and spry two more coats and call it good.

Not sure what brand you are using but with Cardinal they suggest no more than two or three applied thinly per day with amply time in between. Ample depends on temp and if you are using retarder etc. They suggest some thinning and possible retarder as well. I just use the slowest thinner I can get and that seems to work pretty well. It drys to the touch quickly, it is tempting to keep shooting coats but doing so will greatly prolong hardening time.

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Yes, yes and more yes.
1- If you spray coats within 2 days of each other, you don't need to sand to achieve nitro adhesion.

2- If something comes up and I dont spray for 3 or more days, I sand to achieve better adhesion (better safe than sorry)

3- I sand to remove any air bubbles, hair, insects, dust, flecks, stuff, bits and sometimes bobs whenever it needs it. A spot flick of 400 grit under a finger does the trick after 30mins -1 hr.

4- Spray cans don't lay down as flat, so you get more orange peel etc. none of that really matters after you sand it flat and buff it.
..so, i'd suggest (for gloss) spray all the coats you need minus 2 or 3, then sand flat to 600, then your last coats. wait to cure n buff

If satin/matt, same thing but sand to 400 then final 2 coats.

Not a Ukulele but I painted my first guitar with Guitar Reranch Surf Green and a can of Reranch clear. I did not sand between coats. It was orange peeled when I finished painting. I let it gas off for a month and then wet sanded up to 2000 grit. I then used McGuires swirl remover and then McGuires polish. This is how it turned out.

Just a big thank you to Allen, Beau and Pete, who persuaded me to level sand. I sprayed 6 coats over 2 days, then went on holiday for a week, came back and level sanded. I've now added a further 6 coats and it's laid down much flatter than I expected. So it does make a big difference. There's a couple of little areas on the top which need a drop fill, so I'll do that and wait a week again and level the said areas, before giving the top one final coat.Cheers Mike.

Hello everyone, I have an interesting situation here that i'd appreciate some advice on...

I have four koa ukulele that I'm finishing with nitro lacquer. I sprayed the first 5 build coats with a 50\50 mix of thinner lacquer, and let it cure for a week.

I level sanded dry with 320, then lightly with 400 using a semi-hard rubber/foam block.

I then sprayed another 5 coats of 50\50 lac\thinner.

The day after this last spray session, I'm noticing scratches in the finish from what I assume were created by the level sanding. These scratches are only visible in certain lighting, but they are definitely there. I am 100% sure that they are not in the wood.

My question is, do I allow the finish to cure the required three weeks and hope that they wet sand out? Not sure if this is normal or I messed up somewhere in the process?

Thanks!

I don't use nitro, but I use shellac and I think the situations are basically the same when it comes to sanding out. Using 400 grit to level sand seems a bit coarse to me. I use 800 or sometimes 600 if things are looking gnarly. 400 removes a little more material than is maybe needed. But in the end analysis it really doesn't matter if you have enough material. All scratches will get smaller as you increase your grits (800, 1000, 1500 etc. etc.) until you achieve the degree of shine you want. You just don't want to go through your finish which = not good.

Thanks, I've never used shellac other than as a sealer, but for nitro, p800 seems way to fine for the initial level between build coats. My fear is that the scratches I see are down in the finish, and my final level and buff will just polish the surface over them. Perhaps this is just normal, I just would rather fix it now if it is a problem