Sealer Coats with Nitrocellulose Lacquer

Introductory advice on using nitrocellulose lacquer. December 14, 2009

I am new to spraying lacquer and wanted some advice from those of you who are more experienced than I am. I'm spraying a nitro lacquer and wanted to know if you must use a sealer, and if so what type? How many coats of sealer? Also how many coats for a durable finish? I have sprayed water born poly and some lacquer but never with what I consider to be professional results. Any good advice would be appreciated!

Forum Responses
(Finishing Forum)
From contributor N:
I pretty much use (and have for 35 plus years) a lacquer sanding sealer all the time as it is simply part of a system engineered by the manufacturer. Sometimes, however, I use shellac as my sealer if there is a chance of contamination from a silicon based polish used on the furniture sometime in its life. If you are finishing new wood then just go ahead and use the lacquer sanding sealer. Sometimes I will spray a coat of lacquer sanding sealer, abrade with either 3M 320 Tri-m-Ite sandpaper or maroon Scotchbrite and spray on another coat of lacquer sanding sealer if I feel I've sanded through the original coat like on the edges or corners or etc. I generally use a semi-gloss lacquer over the sealer and put on two or three coats of semi-gloss lacquer. The beauty of lacquer is that it chemically melts itself into the previous coat so you can put as much (within reason of course) semi gloss lacquer as you want. Spray, polish with successively finer grits of abrasive then spray semi-gloss again until it pleases your eye.

From the original questioner:
Thank you for your response. I donít have time to hang around others shops to learn and I think most would chase me away. Thatís the situation Iím in. I have some large commissions (for me) and I want to learn the finishing aspects of the business (the construction of the cabs I'm good on). This has been frustrating for me but Iím a fast learner and have no fears that I will do fine once pointed in the right direction.

From contributor F:
The short answer is that you don't need to use a seal coat. But there are times when it's advantageous. The easiest sealer is just to use the same product youíre spraying for a topcoat. Thin it down a bit and spray your first coat. Then successive coats go on normal. Lastly I'd say if youíre looking for a very durable finish, NC lacquer is not it. A pre-cat will get you a little more durability but it really depends on what youíre spraying.

If youíre just starting out spraying I'd advise spending some time with WB products as that is where the future of spraying is gradually headed anyway. The new generation of WB products is better in my opinion than the NC lacquer anyway. I've been playing with Aqualente and aquabarnice and find them to lay down and leave a nicer finish than the Magnamax.

From contributor G:
Sealing allows you to sand off the raised grain and nibs before the money coats. For what you are doing, nitro lacquer is fine. You can use thinned nitro for your sealer.

From the original questioner:
Thank you for your responses. The reason I'm using nitro is that itís easy to use and it's a common finish here in Israel. I agree about the water based products being "the wave of the future" It's just that I'm not there yet. I'm also using the Nitro because I'm trying to match an existing finish and since I donít spray all that much I don't want to waste a lot of lacquer thatís mixed and just sitting in the gun cup. I'm going to enquire of my supplier about a sanding sealer and give him a talking to (in a nice way) about being more open with information sharing.

From contributor K:
If I may weigh in little late: lacquer sealer is basically standard lacquer with stearates added, which allows the wood nubs that are created when adding finish to bare wood to be sanded easier without gumming up the sandpaper. Finish lacquer could be used in place of sealer, but sandpaper usage will go up. I typically use two medium-wet coats of sealer thinned 50/50 with lacquer thinner) and let dry. Then I sand off the nubs using stearated 320 grit sandpaper, backed by a chalkboard eraser and using light pressure. Then I spray on two, sometimes three coats of finish lacquer, again thinned with lacquer thinner per manufacturer's instructions. It may be necessary to do light scuff sanding (320 grit) after the first finish coat.

Nitrocellulose Lacquer is still my finish of choice. As it has been in use since the 1920's, all of its foibles are well-known and easily resolved. Often, a given project will alter the process or technique that I use, but I always start with the basic approach first. I am still undecided about pre-cat. Although it lays outs for me beautifully, I have heard (unconfirmed) "stories" about how it acts in the real world of customer use and mis-use. I have tried water-based products and I am not ready to move to them yet. They are too sensitive to humidity. Just call me old-fashioned. Naturally, a group of 50 refinishers will have 50 individual techniques and products that they swear by. Hope this provided a basis for you.

From the original questioner:
Thank you. You are not too late as I am always learning and prefer to learn from others more experienced than I. I do however have some additional questions. How many finish coats I should build for a durable film? I know this can vary from project to project. How long for it to cure out (not gas off)? I should add that am using a gloss lacquer.

From contributor K:
The answer to everything regarding finishing is, "it all depends." On the specific finish material, what method is used to apply the finish, one's own personal technique, whether the item being sprayed is vertical or horizontal, what the item is being used for, and maybe 37 (smile) other factors. The short answer is that I usually start with the two coats of sealer (at 50/50), follow with a minimum of two coats of lacquer (at 50/50). If the piece is a tabletop or other heavy-use item, I will add another coat. If the tabletop is going to be rubbed out, I will add several more coats, which will later be sanded and rubbed out.

Points of consideration: if you thin 50/50, one-half of the material being sprayed will evaporate, so a 4 mil (medium-to-full wet) coat will dry to 2 mil. Other thinning ratios will leave more or less finish on the item.

Usually an airless spray rig will put out a higher volume of liquid than others types. So, technically, it is more a matter of how much final finish material remains on the piece, rather than the number of coats. On a practical note, I perform my basic schedule; if the result is what I am looking for, I quit.

If you are spraying on a horizontal surface, you will usually put on a heavier coat than on a vertical surface, which is usually ok as vertical surfaces usually have a lower wear-factor.

Curing time - a substantial amount of gassing-off will occur in the first few days; most consider that it takes about 30 days to be fully cured. This will depend on the overall thickness of the coat and how heavily each coat was sprayed. On a practical note, I would be ready to deliver a bookcase in a few days; a table in two-three weeks after spraying (if I am not doing a rub-out).

Filling and rubbing-out will require a different time-table.

As I am sure you know, a high-gloss finish will show up any imperfections in the wood (small dings, depressions, etc.) or the finish to a much greater degree than lower sheens.

Although the design of the finish materials are very much based upon hard science, the use and application of these same materials is based more on technique and craftsmanship, augmented by knowledge of how things are "supposed" to work.

From contributor A:
I have a shop in the state of Washington so humidity plays a part in finishing. I use a Diaphragm airless sprayer in my production. I use SW products - two coats of hi-build pre-cat lacquer and one coat of sanding sealer. The purpose of sanding sealer is to seal and raise the grain. If you put on a full wet coat your sanding should be a very light scuffing with lacquer sanding pads. The idea is to just knock off the imperfections. Then apply the full wet top coats. Some shops use the two coat hi-build without the sanding sealer, but in my opinion the results are not as good.

From the original questioner:
Thanks, it has been my experience too that raising the grain and sealing help improve the overall quality of the finish. I experimented with the sealer, first using none than applying two thin coats and lightly sanding. After an application of two or three coats of lacquer it looked good but I was curious to see if more is better or will offer a more durable finish of will it become too brittle, so on and so forth.

From contributor K:
I need to correct a statement that I made earlier where I said: "If you thin 50/50, one-half of the material being sprayed will evaporate, so a 4 mil (medium-to-full wet) coat will dry to 2 mil." This is patently not true and I don't know what I was thinking. I was trying to make the math easy and got lost in the explanation. I'm going to try again, without getting into the math.

The final thickness of nitrocellulose lacquer on the piece is usually suggested to be about 3 mil.

The liquid portion of the lacquer and the added thinner will evaporate, leaving only the solids. Typically, the solids content of (unthinned) lacquer can range from 18%-30% (as a percentage of volume). So, if spraying the unthinned lacquer were an option, everything would evaporate except the 18%-30% solids. Add thinner and a lower quantity of solids per coat will be applied, ergo more coats to achieve the same results. But this is trying to address the "science" of the process. The real world method says that in most instances, two coats of thinned sealer, plus two-three coats of properly thinned lacquer will put a sufficient thickness on the piece.

In troubleshooting electronics systems, a good technician quits when the problem is resolved. Similarly, for finishing: when the job is done, stop. Once the finish is where it should be, trying to do "just a little more" opens you up to more opportunities for unplanned events. Ask me how I know this.

If you stray too far from that final 3 mil thickness, only poor results can result. Too little finish and the wood is not well- protected (nor will it look its best). Too thick of a coat and the finish is subject to cracking or crazing and may not achieve its intended life span, which can easily be 20 years or more.

Be aware that all of what has been addressed is how many coats it will take to put on a quality finish. Other factors will be involved like the type of wood. Getting a quality finish on oak versus mahogany requires a totally different approach. But that's another chapter.

Do your best, starting with a basis of knowledge. Don't try to over-analyze, until it is necessary. Try to use the same finishing routine each time, as has been outlined by several responders. If possible, get someone who has the experience to critique your work. Self-training can be misleading - if you do not know the "correct" way, how will you know when you are deviating from it?

Have confidence in yourself.

From the original questioner:
Thanks contributor K for taking your time. I originally trained as a chef and in culinary school the head chef almost always sent a dish back to do again even if it was done perfectly! When asked his reason he said ďthe dish is perfect! But you wonít know how to correct yourself when you make a mistake if when in training you donít make mistakes!Ē It was sometimes hard to take but I learned that just because a monkey can drive a truck it doesnít make him a truck driver! I donít intend to be the monkey! I want to understand when I do something correctly, why I did it correct.

From contributor F:
I usually spray a pre-catalyzed lacquer very heavy and spray No Blush on it while it is still wet. Both can be purchased at Mohawk Refinishing. It will dry in about four to six hours. No Blush is mostly lacquer thinner with drying agents and propellants and will also help the lacquer resist and heat damage.