Finishing and the importance of prep for Nitro

[tippie53]

Finishing a guitar is about a few basic things. You have to be sure the prep is done well since this is the foundation of the finish. Any short cuts in the prep and you will be fighting the finish all the way and never get that perfect look.
There are a few misconceptions
1 - You need to sand to high grit
I personally won't sand past 220, That is plenty fine. In most case 180 is good
2 - Sand with the grain
I will sand across the grain on the top. Sanding with the grain can make the top look like corduroy.
3 - Shellac makes a perfect sealer.
At one time shellac was the all around best sealer, but the government now dictates what can be put into the finishes. There are certain solvents used in finish that have been removed. The absence of those solvents will hinder the finish melt in. I find it is best to use the recommended sealers from your chosen manufacturer.
These days it's all about chemical compatibility.

I use Pore o Paq as a filler. Getting a proper fill may be the hardest part of finishing process. I tried water and acetone based products early on and never had much luck, but you should find what works best for you.
You should also follow your Manufacturer's suggested finishing schedule. The techniques used to finish are many but I have learned it is a process and deviation from that process will give poor results. My finish schedule is as follows:

Sand to 180
seal top
mask off back strips
stain
seal
fill
seal
scrape binding remove masking
this gets you ready for finish
then you do the finish in 2 stages
first stage
Spray a coat every 45 to 60 minutes to a build of .016 to .020
allow proper cure time.
THIS CAN VARY BETWEEN MANUFACTURERS
level sand
start with 400 grit and sand to half the depth of the orange peel
then 600 to just about the bottom
then 800 to bottom and level
second stage
finish top coats
2 to 3 coats
allow proper cure time
start sanding with 800 and work to 2500
buff and polish

I can't stress enough that along with the prep, patience is also important; allowing proper dry/cure time between coats. Learning to buff and polish is another skill that takes practice to master. In the end I want to see a topcoat between .010 and .006

I hope this helps. Feel free to add to these procedures as you see fit.

[B. Howard]

Absolutely right John, preperation is key. Most finish failures occur well before the can of finish is even opened. I am actually conducting a presentation on proper finish prep at the Mid Atlantic Luthiers meeting next January. I know that's a ways off, but it was the first our schedules could coincide.

[tippie53]

I will try and be there

[Dan Bombliss]

John,

Out of curiousity, what is the different between using Shellac for the sealer, and using Shellac for the base for a relic job.

Dan Erlewine has a video (Old school, from the 80's or so) On relicing finishes. He sprays Shellac under the lacquer, and then once the lacquer is cured, freezes it and it does the vintage cracking.

I assume the difference is with the sealer it is just a very light coat, and he's actually building. I haven't done a shellac sealer because it made me a bit nervous to mix.

Lastly, I was surprised to hear in another Dan video, he said he doesn't use sealer, and let's his lacquer do the sealing. Things may have changed for him, as far as what he does now, but what's your take on using just lacquer coats to seal with?

Just to add as well, you had stated "I wouldn't sand past 220". I think it'd be fair to a warning going much past 220 risks adhesion failure, especially on resiny oily woods.

-Dan

[tippie53]

One issue with shellac is that the government made manufacturers of the lacquers remove some volatiles that allowed a good melt in and this created issue with adhesion. This created adhesion problems for some of the manufactures . A great finish is also about the chemical compatibility so over time the 2 layers separated.
I had some issues when I first started and stopped using it. Once I went with the manufactured sealer to match the lacquer I got a much better end result.

The sealers seem a bit thicker than shellac and allow a nice build. They are used under and on top of the filler to control stain bleeding. It also allows for the pores to be sealed off and less dips as the finish ages. When I lay out sealer I thin about 10% and do 2 light coats about 20 min apart let set overnight scuff off with a scotch pad fill set over night and seal again with 2 coats. Right after the final seal coat I hit a thinned lacquer coat to assure a good melt in . Then spray finish every 45 min to 1 hr to get a .016 to .020 green coat , this will shrink a good bit in 2 to 3 weeks. I level sand and sand to about 800 to 1000 grit and lay on 3 finish coats, Since this in on a leveled finish I can start the final sanding at 800 grit. Usually work to 2500 buff and polish.

I know some that use just lacquer. The one thing is that you need to allow the proper cure time. I think it depends more on the wood. Rosewood had very pronounced pores and over time the finish will sink into them if not well filled. The filler can also leach out some stain so you can get some color bleed into the finish. It can still work but you need to allow plenty of finish cure time . I don't know if cure or drying is the proper term . Still it takes some time for the shrinking to totally stop.

This can vary by lacquer manufactures. Cardinal seems to be the fastest curing finish I have used. I am liking this product the more I use it.

[Ben-Had]

I just bought a gallon of Cardinal nitro and sealer so I'll be seeing soon how it compares to the Mohawk or Behlens I've used in the past.

[B. Howard]

Most modern lacquers have lost their ability to self seal effectively. Even the unmodified nitrocellulose I use will not self seal. As John mentioned it has a lot to do with changes in chemistry to make these products safer to use. Shellac can still be used as a sealer under lacquer, sometimes. You will need to be responsible in assuring the compatibility between your coating and shellac, or any other non recommended sealer for that matter. And that should involve some impact testing as well as long term adhesion.

Information from even the 90's on finish process should be viewed with caution because even though the label didn't appear to change, the product in the can surely has. These days it is truly best to work entirely within a system to insure chemical compatibility between coatings.

[Darryl Young]

Do the mfgs offer any sealer other than vinyl? If you don't like the look of vinyl, are there no other options from the mfg? Curious.

[B. Howard]

The system I use has a nitro sealer that goes with it so the short answer is yes. In fact they recomend against a vinyl sealer with this product as the unmodified nitro doesn't burn into the vinyl very well. One you can get without a commercial account is a different question, maybe. You would need to inquire with each manufacturer on an individual basis. What is it you don't like about the look of a vinyl sealer?