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And the reasons I don't use them:

https://www.popularwoodworking.com/flex ... -are-they/

IMO, you can vinyl sealers to the same article. I just don’t see the need for them either.

The article states about nitro , if you use it you need sealers.
this is about furniture not guitars. seal coats are there for a reason so you don't get bleed from the woods . Rosewood , cocobolo will need to be sealed. the woods in the article don't refer to this at all.
Todays finishes are about adhesion and final buffing, if you want the flat shine like a Martin you need to seal fill and seal again ,if you want to use a satin or don't mind showing grain sinks you don't need to invest as much time in the finish process

Where many go wrong in finish is not paying attention to chemical compatibility and over sanding the wood. Sand past 220 is going to fine and you loose tooth for the finish to hold on to ,and this is important for a good professional finish. Guitars are not furniture .

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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

A few years ago I had a series of conversations with Philip, the head chemist at Mohawk. He told me that you do not need to use sealer if the concern is adhesion, and that sealers were intended for build coats and ease of sanding. He told me that the most important thing related to good adhesion is making a full wet coat on the first application, regardless of whether that coat is sealer or lacquer. John is right, sealer is needed to stop bleeding.

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It will probably be alright.

Michael Dresdner is one of the foremost experts on finishing - guitars or otherwise. He wrote for American Lutherie, Guitarmaker, Fine Furniture and more. He used sanding sealer. Good enough for me.

At the first Symposium I attended, our group hung out at Dick Boak's Church of Art one afternoon, then adjourned to a bar before returning for supper at Dick's. As Grit Laskin and I walked back, we talked about how much we enjoyed reading and hearing Michael Dresdner expound on finishing. Grit concluded that Michael "understood finishes down on the molecular level".

A few days later, I visited Michael at his Factory 42 location (making Ken Smith Basses) and told him what Grit said. Michael smiled slyly and replied "I understand finishes down on the spiritual level".

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

The question I keep wanting to ask a coatings chemist (funny, I do have a degree in Chemistry ha) is what ingredients are in sanding sealer that improve its “sealing” ability more than the top coat? My opinion is just anecdotal, I quit using ss and vinyl sealers and see no difference.

Sanding sealers add stearates (soap like compounds) to improve sanding and help add "build" to the finish. I quit using them because they seemed to affect the clarity of the finish - they didn't seem to wet out the wood as well, but rather sit on top of it.
Something the article pointed out, among the other "negatives", was that it made the finish less plastic and more prone to cracking. I had not heard that before. On a solid piece of furniture (or solid bodied instruments) where the wood doesn't move too much it may not be a concern, but for acoustic guitars it may be.
For open pored woods it does take more coats to self seal with nitro, but you can get the same "flat" finish, but must allow more time for the finish to fully cure before level sanding.
Whether or not to use a sanding sealer is a personal choice. Often articles only point out the positive aspects of a product, this one shows the negative side of it. Posted for informational purposes only.

I had always understood the stearates were added to prevent clogging of the sandpaper. So, more about the “sanding” than “sealing”. It made a relatively thick coat easier to level than just pure finish. Any truth to that?


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The trick is not to build it a lot, or sand it back to the minimum coverage. It's a bit soft, and everyone understands that you want a foundation to be solid under a hard finish or a house. Too much sanding sealer and your hard finish will dent easier. On spruce I thinned the mixture before shooting. For a mahogany Les Paul, a couple normal coats sanded back to level (and thin).

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

I started using heat lamps in the spray booth that helps with flow out . One thing we have to clear up is not to generalize finish work. There are variables to the process and lets face it
Urathane flows different the lacquers .
I used to use thinners and I use Mowhawk , they are different than DEFT or MINWAX so be sure to understand the material you spray. Here is my finish schedule. I am in an area PA that humidity can swing so I do have a RH and Temp control in my spray room. I use a down draft system .
My thinner is 2 oz per half gallon at 75 degrees and 50% rh. I use heat lamps to get the body to 85 to 95 degrees. I heat my finish to about the same temp. I have heat lamps on my gun so that I can maintain the heat. I use HVLP guns all gravity feed and I have nozzles at 1.3mm . I am using about 18 lbs and each gun I use has a regulator on it. I also use a fugi semi pro. My sealer is the Mowhawk Vinyl sealer and finish is the instrument lacquer.
I filter the material as I fill the cup. Adjust the spray to about a 4 in oval. I also have my body on a rotisserie so it spins. I use tape under the fret board extension so I can measure finish build. I will set 3=4 small pieces of tape that I can pull and measure during the spray cycles .I spray 2 coats of sealer , allow 3o min between coats, Light level sand then apply filler ( AQUA COAT ) I may color the filler. I apply it with a card and do 2 applications. BE SURE TO READ THE MANUFACTURES INSTRUCTIONS . after this dries I level sand lightly with 400 grit and apply 2 coats of sealer , allow dry time and level sand with 400 600. Then I apply 4 coats. This I let set for 2 weeks then level sand. About 2-3 days after initial spray I will look for drop fill areas. I level sand using 600 1000 1500 2000 2500 3000 wet sanding. Then buff and polish If I blow through I will apply 2 more coats and allow cure time.
My buffing process is mostly Mazzerna and if I need maquires swirl remover as my final if needed. I have also used 3M finesse it
The most common mistakes I see with people starting out is applying to heavy and not level sanding enough. I usually find my finish at the bridge to be in the .006 to .008.

_________________
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

Sanding sealer and vinyl sealer are two entirely different materials. Lacquer-based sanding sealers are loaded with sterates and silicates to quickly fill pores, build the film thickness, ease sanding, and have undesirable impacts on both film hardness and topcoat adhesion for instrument use. Vinyl sealer is a vinyl-loaded lacquer barrier and tie coat, designed to be applied at well under a mil final thickness without any bulking or filling components.

While both shellac and vinyl sealer provide both barrier coating and tie coat, vinyl sealer does a better job keeping dyes in fillers or unfilled woods from bleeding into subsequent lacquer shader/color and top coats. Vinyl sealer also seemed to do a better job for us of preventing delamination due to sharp impacts - a common issue with instruments that may not really matter much for furniture builders. We still used shellac for sealer on restoration work, but went with vinyl sealer on nearly all lacquer body finishes (EnduroVar on most necks...much better finish in terms of wear and feel than either lacquer or precat lacquer for that application).

Concur with the full, wet coat application of both lacquer top coats and vinyl sealer over bare wood, but of note for Mohawk classic instrument lacquer: we found that with the latest formulation change (now about six years back, if my recollection of the discussion is accurate), we needed two days to shoot all the top coats. Applying all of them in a day for a final .0035"-.004" film thickness (thinner on tops) as we had once done resulted in the same stress-crack issues near bridge wings that Martin saw with their finishing system in the 2010 time frame (not Mohawk, but subject to the same sort of lower total VOC formulation pressures pushed by EPA and some state-level surrogates). Not sure what Martin did to address their stress crack issue beyond dramatically curtailing their insanely generous lifetime warranty, but breaking up top coat application fixed our issues without requiring the addition of film-softening butyl cellosolve or other materials with undesirable long-term drying impacts.

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For the times they are a changin'

- Bob Dylan

Woodie have you tried using heat in the booth? I have been doing well at 4 coats about 1 hr apart. Martin isn't using mowhawk .They also are using a propriety process. They do apply heated lacquer and it is all in a controlled room with automated applications. From the booth it goes into a "clean room" and they seem to have fixed the cracking issue.
If you spray it pays to read the labels and use as directed as there is chemical compatibility as well in this process.

_________________
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

When I sprayed acrylic lacquer in the winter I warmed my lacquer (and sealer) in a couple inches of water in a Crockpot.

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

Clay S. wrote:
"Something the article pointed out, among the other "negatives", was that it made the finish less plastic and more prone to cracking. I had not heard that before. "

It's a violation of the old artist's rule of 'fat over lean' If you have a softer coating on the wood the hard later coats can't move with it, and end up cracking. An extreme version of this is that 'crackle base' they sell for artificially antiquing furniture. Basically it seems to be latex paint base with no pigment; it dries to a fairly thick and flexible film. Almost anything you put over it is pretty much guaranteed to crack and check. Artists use more resin and pigment in the base layers of a painting, and mix in more oil as they go along to make the top coats more flexible. Over time, of course, the oil in the top coats hardens up and shrinks, but in general doing it that way probably helps the paintings hold up.

It seems to me that the real trick with any sealer or filler is to use it to fill pores, but sand back to the wood before you put on the final finish. That pretty much avoids shrinkage/flexibility issues and also the differences in refractive index that cause 'veiling'. And I agree that 220 sandpaper is fine.

one thing that should be noted is not all lacquers are the same
Minwax Deft are not instrument grade rattle cans are not but many use them
The more finishing I do the more I learn and many thanks to those that shared their information.

_________________
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

Actually, the recommendation to break up the top coat finish sets was based on discussion with Mohawk's technical staff. No need to sand between the sets, but allowing the excess VOCs to cook off seems to have done the trick.

_________________
For the times they are a changin'

- Bob Dylan

For pore filling...sanding sealers have been abandoned long ago for me. I choose modern, two part polyester pore filler (Simtech) because I can level the next day and never worry about it falling back into the pores. From there I can spray nitro OR a two part poly with the properties of nitro. The adhesion with nitro over poly is pretty good...but adding a vinyl sealer coat over the poly sealer is the REAL DEAL with respect to adhesion.

That extra vinyl sealer step is why I use the poly top coat (same hardness as nitro). I can level the finish within three days of bare wood. The POLY top coat binds molecularly with the sealer. The nitro does not.

Plus, the optical clarity of the polyester sealers is particularly good. So, the polyester and urethane systems have evolved to be MUCH easier, faster, to use...and they add adhesion and clarity to boot.

Plus, with polyesters you can add or subtract the amount of thinner (acetone) for the sake of processing options. This alone made my decision. I can thin it to whatever I like after combining the resin and catalyst.

Plus, it doesn't bubble under or skin over.

Plus, I can add more or less catalyst for a faster or slower reaction (up to a point).

BTW....I did several million test panels to determine all this. Controlled impact tests as well. Once I had done the first whole guitar using polyester, I never went back to a previous system. This was the first guitar I did with the full poly system.

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I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

we should start a topic on finishes per se
not all finishes are done the same and to newbies it can get confusing fast.

_________________
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

It's been pointed out to me, that I have used different sealers in the past other than sanding sealers. It's true - for a time I was using DuPont 1980S sanding sealer, a clear, water thin sealer that basically acted like shrink wrap in a can. I started using it back in the mid-80's when refinishing a 1962 P-bass body that would begin crazing after the 5th or 6th coat of white acrylic lacquer (DuPont). I never did figure out what had contaminated it, but after the 3rd time experiencing crazing of the lacquer, I asked my supplier for suggestions. The 1980S worked like a champ - 2 thin coats, scuffed lightly with red Scotchbrite and no more crazing problems. I've seen the bass recently, and it still looks good. And I don't think I have any more of that 1980S sealer, having used it up many years ago.

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