Any problems with spraying Waterborne finishes?

[jim gossage]

I am looking for advice on waterborne finishes. I am in the process of building a dedicated spray booth. I had initially planned to set it up to allow spraying of oil based lacquers with an explosion proof fan, but in doing some reading on SMC I came across repeated endorsements of waterborne sprays including a lack of downsides. "Why would you spray OB." When I first set up my current booth I bought a QualSpray gun and EMtech8000. I recall reading (maybe incorrectly) that waterborne sprays need a shellac or other sealant as first coat to avoid raising the grain (for some woods, ?plywood), and that sounded like a negative to me. I have sprayed a number of mostly small projects (picture frames, clocks, etc) and have settled on an initial sanding prep using 220 grit, then 3 coats of EMtech8000, then sand with 220 grit, then a final coat. Sanding in between other coats has not seemed to make a difference for me. This gives a good but not great result. The finish is not as silky smooth as it was after my initial sanding prep. And the finish is not as smooth as when I brush on 3-4 coats of OB poly. Thus my interest in a spray booth that can handle OB.

My questions: 1) Are there really no downsides for the average hobby woodworker to spray WB, and 2) what am I doing wrong with my current EMtech spray process? Thank you.

[roger wiegand]

I'm not sure what an "oil based lacquer" is, but the dry time of any oil containing finish is a serious drawback, especially if there's any dust in your finishing environment. A nitrocellulose type lacquer eliminates that problem, and seems to be a wonderful finish to work with at the cost of having to have a serious spray booth and ventilation system. NC lacquer seems not to be nearly as tough as the available WB finishes. I don't have a spray booth and often want a more robust finish, so use the Emtech products with added crosslinker. I'm partial to the EM6000, but for electric guitars and such use the 8000. Things that don't get handled much I finish with shellac, either brushed or sprayed as often as not, as it's so easy and looks so good.

One change I'd suggest to your schedule is to wipe down the workpiece with a damp rag prior to the final, very light sanding with fresh sharp paper. This raises the grain and allows you knock off the "fuzzies" so they are not problematic later.

The other thing I do with pretty much all finishes, whether oil, WB, or shellac is to allow them to throughly cure and then rub them out to the desired final sheen with anything from 0000 steel wool to pumice/rottenstone to automotive polishes. I find this improves the appearance dramatically and incidentally eliminates any nibs or imperfections. It also lets me keep only gloss finishes, reducing the clutter in the finish cabinet. Not sure I'd want to do it for a whole set of kitchen cabinets, but for my very low volume furniture and instrument production it's a very worthwhile investment of time/effort.

[jim gossage]

Thanks Roger. Sorry for my inaccurate use of the term "oil based lacquer" - I should have just said oil based finish. Please say more about the damp rag wipe down. Is that done before any finish is laid down or before the final coat? Do you let the piece sit damp for a few minutes or sand immediately?

[Curt Harms]

Spraying dewaxed shellac goes pretty quick and you can add color via Transtint if you wish. That does require sanding but my experience is that sanding shellac is pretty easy and very quick cure (not really a cure, just evaporation and alcohol evaporates pretty quickly) so it's not a multi day process.

[Jim Becker]

Waterborne finishes are generally designed for spray application. They dry quickly and overspray is often dry or nearly dry before it hits the floor or surface under the workpiece. The same is true for solvent based finishes like lacquer as well as for shellac. These are all suitable for spraying. Waterborne finishes (and to a certain extent shellac) are much safer to spray than solvent based lacquers and 2K finishes that have dangerous chemicals.

Oil based products are generally not the best to spray because they dry slowly and that means a sticky mess and harder cleanup. Oil based finishes hand apply nicely and because of the long dry time, tend to level out well to help eliminate any evidence of hand applying if one does it correctly.

[John TenEyck]

You don't need to spray a coat of dewaxed shellac before applying WB finishes, but it definitely helps eliminate raised grain. The type of wood you're spraying on makes a big difference, too. I had some white oak that I needed to sand to 325 grit before I could raise the grain w/o creating a very rough surface. Maple, on the other hand, doesn't need more than 180 or 220 grit. In the end, I have not found a WB finish that totally avoids grain raising, regardless of what I do for prep work, so I always spray a coat of shellac (except for painted surfaces in which case I spray a primer) first unless the color shift cannot be tolerated. If I want a dead clear, colorless finish, for example, I can't first use shellac because it will tint it amber/red.

With respect to your finishing schedule, I would sand to at least 220 grit, then raise the grain, then lightly (and I mean very lightly) sand with 325 or 400 grit. You can raise the grain several times, as well, if you get a lot of fuzzies the first time, because sanding them off creates more area where the grain can raise when it sees water again. After grain raising, or spraying a coat of dewaxed shellac, spray your first coat of WB. Make it a light coat, it's OK if it doesn't completely flow out. You want to seal the wood, not try to put a final finish on it yet. Let it dry at least an hour and then very lightly sand it with 325 or 400 grit, to knock off the nibs. It shouldn't be really rough. If it is, you didn't do a good job prepping it. However, if it does happen, you can sand it back until it's smooth with 325 or 400 grit, and spray another coat. After 2 or 3 coats, sanding the minimum amount to eliminate the roughness between coats, you will have a smooth surface upon which to spray the last couple of coats.

WB's are not quite as friendly to use as solvent based finishes. However, they have so many advantages that the extra effort makes it worth, at least for me. From my perspective, NC lacquers are not nearly as durable as good WB finishes, so there's no performance advantage to using them. There is higher performance, catalyzed, solvent based finishes, but they are at least as dangerous to work with, and some much more dangerous w/o the appropriately outfitted spray booth and PPE.

WB finishes range from water clear to solvent based look a likes and are easily tinted to get any tone desired. Many are just as transparent and brilliant as solvent based finishes, EM-6000, and GF's Enduro Pre Cat WB lacquer, for example. EM-8000 is exceptionally durable if you add the cross linker and allow it to cure for several weeks.

Don't give up quite yet.

John

[roger wiegand]

The grain raising with water is done on raw wood before any finish is applied. It makes for a nicer finish with solvent-based finishes as well, so I always do it.