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I've read in various books* and on-line the virtues of a "flow" coat that's 1 part lacquer to 4 parts thinner after the "final coat" is finished. You guys that do a nitro flow coat, do you recommend it?
Do you have recommendations for a flow coat?
My technique is 95% run free, but I'm a lot apprehensive to shoot a 1:4 flow coat.

BTW,
This project, my second acoustic guitar, has its "final" coat of Mohawk SIL (Stringed Instrument Lacquer) yesterday.
As my spraying equipment (StewMac conversion jam gun and nailer compressor 3.2 CFM) is on the way side of budget, my finish is not really smooth. It's not bad I would like a smoother finish to wet sand, and it is, afterall, another skill to learn. Just wish I didn't have to learn it on MY guitar




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*Spray Finishing Made Easy; Jewett
Complete Illustrated Guide To Finishing; Jeff Jewett
Understanding Wood Finishing: How to Select and Apply the Right Finish; Felxner
Guitar Finishing Step-By-Step: Erlewine
Etc.

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Measure Twice,

Karl Borum

I used to do it, and it worked.

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"Act your age, not your shoe size" - Prince

as with most things is the universe it comes with a cost...and that cost is reactivation of deeper coats of the finish...e.g. you will increase the time it takes to harden and get to a state sufficient to buff out...

but yeah, as Chris noted, it works

If you intend to walk away and leave the lacquer build as-is, yes a flow coat will make it look better. But on an instrument, you’re likely wanting to level, wet sand and buff after a month, so a flow coat gets you nothing. Nitro is very forgiving, although good application makes levelling easier.

The important thing I found with Mohawk is getting it to flow well from the gun. The Mohawk reducer is very “fast”, and they don’t sell slower versions. Don’t be tempted to use more than 15% reducer, and only if your gun needs it. Slowing it with 10-20 ml per litre of butyl cellosolve helps flow a lot.


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Thanks, yes, I would expect easier leveling.
I guess I'm really asking, how difficult is a flow coat to spay without runs? I've never sprayed lacquer thinned 4 to 1 on a guitar.

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Measure Twice,

Karl Borum

I wouldn't do it if you are sanding and polishing later. It isn't needed and having a run at this stage could be problematic.

If you have already applied enough lacquer at this point I wouldn't bother trying to get it to flow out better.
On the next one you might try "double coating", by first spraying a light coat, letting it flash off and then spraying a normal coat on top of it. It allows for a quicker build and can sometimes give better flow out.
If you have enough finish on the instrument you can start with a coarser sand paper to knock down the orange peel quickly and then move on to a higher grit.

"StewMac conversion jam gun and nailer compressor 3.2 CFM"

You could try the $15 high pressure jamb gun Harbor Freight sells. High pressure guns usually atomize better than HVLP guns and often use less air than conversion guns. I have used a similar sized compressor to spray guitars with the HF gun and gotten reasonable results (still had to sand and buff). It would be a $15 gamble that may or may not payoff.
Sometimes it is the way you spray. My own simple technique is to spray things "flat" as much as possible. For guitar bodies I mask off the bridge and fingerboard extension areas and attach a couple of blocks to the tape. I put the body on something similar to a rotating cake stand. With the body laying flat I will spray the soundboard with a full wet coat and the sides with a lighter coat. After it flashes off I will flip the body over on to the blocks and spray a full wet coat on the back and a second light coat on the sides. The top and back are getting heavy coats that would "run" if the surfaces were vertical and the sides are being double coated. With a wet coat the solvent takes longer to leave and there is more time for the finish to level itself. People that have better technique can hang instruments and get nice even coats and no runs. I have had mixed success with this and find spraying flat more to my liking.

The goal is to build enough finish so that your final finish will only be a few thousandths thick after leveling. Knowing how much finish I’ve built and how much will be left after leveling is still tricky for me.

I took Robbie O’Brien’s online finishing course, and he shoots a flow coat after the final level. I don’t understand the benefit, but it seems in line with the courses you mentioned in the OP.


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Well, if you saw it on the internet it must be true, right?

I think I learned a thing or two in 20 years.
First what sprayer are you using? I use a semi pro 2 fuji . I swapped out the needle to a 1.0. My best advice is the prep. If you don't do a good prep you are never getting a great finish
I don't sand past a 220 grit
stain if you wish
now your ready to finish
Decide on your filler
I use aqua coat and lmi's water based filler
if I use a colored filler A will have to scrape bindings
so
A stain
B seal ( i recommend to use manufactures suggested sealer ) 2 coats light level sand
C fill
this process can vary to what you are using. I do aqua coat I use a vinyl glove and rub it in. allow about 45 min level first coat with 220
apply 2nd coat level off with 320 apply 3rd also 320
D wipe down with naphtha allow 20 min dry and blow off
E seal coats 2 level off with a light rub of 400
F build coats. I use mowhawk finish My mix is a 50 50 mix of retarder and thinner for better flow and mix about 50 50 to 60 40 depending on temp and RH my booth has climate control, I use a rotisserie https://www.youtube.com/watch?v=escxSMDoC5g I apply 6 to 8 coats on the first day. I allow about 5 to 7 days to cure . without climate control this may take 2 weeks. You know your ready when you can sand with 600 drill and it doesn't pill up on the paper.
G level sand process is wet sanding I use a festool sander 125 and klingspore 600 800 1000 and then to P1500 P2000 P2500 P3000
buff and polish
if you sand through apply 3 more coats allow cure time and them level off stating at 800


this elinimate runs.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Ha ha Barry. Guess you missed the book list from respected craftsmen in the field that refer to a flow coat?

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Measure Twice,

Karl Borum

My last couple of coats I thin a little more, instead of my usual 10% around 20-25% plus another and add another 10% retarder. Makes it a little easier to sand before buffing-less orange peel.

I level throughout the spray sequence. By the end it is as level as I can possibly have it. I then shoot a 50/50 mix at the end for a flash coat. Let it hang for about a month and then polish, usually by hand.

Thanks to all for the useful information.
With a 50/50 or thinner flash coat, do you still put it down as wet as a build coat? Guess I should do some sample panels to see how easy these flow coats run.

As the lacquer dries over a two week period it seems to get exponentially harder, and harder to sand. I would like to reduce the amount of time on the final sanding process with the flow coat. I've only done one guitar, but I spent probably 20 hours wet sanding before buffing, after a nice, glossy, wet final thick coat.

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Measure Twice,

Karl Borum

20 hours! It gets quicker as you gain experience but that is way excessive. I probably sand for 2 hours after the final coat though I do sand twice during the build process.

I thought the same. My arms fell off just thinking about it. Kbore, is that a typo?

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If you are sanding for 20 hours you should probably be starting with a coarser grit. If I get much orange peel I will start with 320 and a "hard" sanding block to cut the tops off the orange peel and level the finish. You can make a "hard pad" for an orbital sander by spray gluing a disc of Formica to a worn out hook and loop sanding disc and then spray gluing a new piece of sandpaper to the smooth formica surface. This allows you to use a R.O. sander for initial leveling (don't get carried away - just knock off the high spots!)

FWIW, I level my nitro with 600 grit and my Festool ETS-125, then 1000, then buff. Menzerna P204 followed by P175. I don’t remember what kind of wheels I have. There was a thread here on these compounds a few years ago, and that’s when I picked them up. If you find the thread, you’ll know exactly what I’m doing because I just followed the advice.

Most people level with 400, but I’m just concerned with sand through and prefer the extra work just to be careful.

Leveling is time consuming. After that it isn’t too bad.


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I started out with 3M P800 wet or dry last build.

Last build, I started with my old lifetime supply of 3M 600 wet or dry, ran out, bought some new "600", labeled P600.
The P600 was way coarser than the old 3M 600 that I had, you could see clearly side by side on plexiglas that P600 was WAY more aggressive than the old 600 I ran out of.

I stopped and replaced it with P800 which was a similar scratch pattern to the "old 600".
That's how I learned about CAMI and P graded sandpaper.
So, I could say I started out with 3M P800 wet or dry.
Maybe P800 is a bit too fine to start with, I'm gathering?

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Measure Twice,

Karl Borum

Here is a handy chart for comparing cami, p grades , micron abrasives:
https://abrasives-south.com/resources/a ... ng-system/
It will help you see where the two systems diverge.
If you have a nice level coat P800 should be O.K.. If you have a lot of orange peel P400 and a hard pad might allow you to level the finish a bit quicker.

I keep it simple. P1200 for level sanding and then on to the buffer with 4 different grades of compound. This is with Cardinal lacquer allowed to dry/cure for at least a month.

I love the P800 sandpaper but I think even it is too coarse to start final level sanding. The chart shows it is equivalent to CAMI 400. It is good for between coat sanding but not for final. It makes deep enough scratches that you have to sand a LOT to remove them using the next grit.

On occasions where I've had significant orange peel I've used 320 grit (P400) to knock the tops of the orange peel off, still leaving the "valleys" untouched. The sanding scratches will be removed when the next finer grit touches the "valleys". The key is to move to the next finer grit when the last one has given a uniform scratch pattern.

Thanks for all the replies, much appreciated.
I'll save the flow coat for a day when I have better equipment.
Don't have orange peel so my plan is to start with the 800 to knockoff the peaks, move to 1200 then 2000 . I used 3000 last time, it went really quickly, I don't have to decide today weather or not to 3k it.
Thanks again!

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Measure Twice,

Karl Borum