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!
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.
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.
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.
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.
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.