Waterborne Finishes – Pro and ConPosted on May 9th, 2016 By Charles Neil No comments
My beginning was in automotive finishing and it remained my career for the next twenty years, then I switched to woodworking and finishing. As with most finishers, I began with the old nitrocellulose lacquers and while they were very friendly to use, they were less than a long-term hard use finish. Then came the pre-catalyzed and post-catalyzed lacquers, a big improvement and they remained relatively user friendly. With the introduction of urethanes and other manmade resins, durability improved even more, but they also had their issues.
Even with modern day post-catalyzed conversion varnishes and so forth there are environmental issues. Changing formulas to meet VOC regulations or in many cases states simply not allowing them to be used, solvent-based products have a very uncertain future.
Today, in my world, I make high end custom furniture and teach finishing, predominantly to hobbyists and semi-professionals. These finishers do not have an environment that allows them to use solvent-based products, especially if it is sprayed. So I began my journey into the world of waterborne finishes. I’ve converted all of my finishing to waterborne products and am glad I did and would never consider going back to solvent based.
At the beginning of the journey, I listened to all of the experts and to be honest, the results were less than desirable. The heavier viscosity of waterborne finishes had everyone saying that you had to use larger needles/nozzles. What I found was that no matter what I did, atomization of the fluid was not good. Reducing the amount of fluid at the gun helped a lot, but I just couldn’t get the ‘butter smooth’ off the gun finish that I was used to with the solvent-based finishes.
So, I closed the books and turned off the computer and it was the best thing for me because I began experimenting with various guns and needle/nozzle settings as well as product viscosity.
Waterborne finishes are typically much thicker and much higher in solid content and since the resins are not dissolved as in a solvent finish, the finish tends to spray ‘coarser’ and is harder to atomize, it also lacks the flow in of any minor overspray. This leads to a gritty feeling finish. It didn’t matter which gun, cup or system I used. I got the same results with the gravity fed, pressure fed, bottom cup, compressed air or turbine driven.
Then, through experimenting I found that dropping the needle/nozzle size to a 1.4 or 1.5 and increasing the fluid, I got a much better atomization and with the reduction of the waterborne finishes, usually up to 10% is allowed. However, it’s still water and water has surface tension, the reason bubbles of water stand on your car or a glass of water can be slightly overfilled. It’s also what allows ‘skeeters’ to walk on water. It’s this surface tension that the smaller needle/nozzles help to break up.
Thinning of the finishes too much can be detrimental. Unlike solvent, what actually seems to happen is that the molecules of finish, which are suspended in water and as stated, not dissolved, tend to be forced further apart. Not only can runs and sags be far more prevalent but the finish simply doesn’t lay out as well.
To understand this, think of a waterborne finish as a zillion tiny BBs on a surface, they have to come together and bond to become a film. The further separated they are, the coarser the film. While a 3% to 5% reduction can help, much over that and I have issues. My standard needle/nozzle is a 1.4 or 1.5, my general rule for viscosity is quite simple. If I can pour the finish in a medium mesh strainer and it flows out at close to the same rate, I’m good to go. While this technique may seem quite antiquated and inaccurate, it has never proven me wrong.
In the case of the smaller needle/nozzle I find that by adjusting the amount of fluid and pressure, I can have a waterborne finish that sprays like a solvent and the results are the same.
Recently, I got my hands on Apollo’s AtomiZer gun. It is a pressure fed, bottom cup gravity fed or pressure line fed and can be used with either compressed air or turbine. I often noticed I could get better atomization from a gravity fed, especially one with a larger cup and the fuller the cup, the better. It was simple, the added weight was helping to push the fluid to the smaller needle/nozzle.
It’s the same basics that allow you to put your thumb over a water hose and spray water, but if you have low water pressure, it’s not a good break up, higher pressure and you can get about any degree of break up you want.
So, I began to look for pressure fed cups, typically what I found was the pressure was limited to bottom cup guns and then only enough pressure was afforded to help lift the fluid. To help eliminate the higher pressures required by the old siphon guns, they had to draw the fluid from the cup by suction created from a large volume of air moving through the gun.
Then enter the AtomiZer, problem solved. Apollo finally figured it out. Waterborne finishes, push that finish through a smaller orifice and it sprays. Much like an air assisted airless or the commercial sprayers for paints, it’s that added finish push that will rewrite how waterborne finishes are applied, not to mention the precision of the gun. While many systems will spray waterborne, this technology that Apollo has incorporated will change anyone’s mind who has ever pulled the trigger on a waterborne finish.
Waterborne FAQ and Issues
You have to be careful of any finish left on a tip between coats, it sets up hard and fast. Once set, some M.E.K. is all I have found to clean it. I simply drape a wet cloth over the gun tip between coats.
You want to use a Teflon lined cup. Nylon isn’t bad but with aluminum and steel, the finish likes to hang on the sides and can semi-setup and contaminate the finish.
Waterborne finishes are typically water clear and afford no slight amber tint or warming of the finish like a solvent base finish does which is more of a reaction of the wood and the solvents. I find a quick seal coat of 1 lb. cut of either a blond or amber shellac cures that.
Waterborne finishes are more expensive but when you compare solids content it isn’t really. Add to that fire insurance, employee workman’s comp insurance, and the whole environmental issue, not to mention hazmat shipping, it’s actually cheaper.
Overspray, while minimal can be an issue in multi-piece application. Unless you have adequate air movement to remove any airborne particles, you can get overspray settling in the finish and because of it’s quick set, it will ‘grit’ the finish. My solution is to spray multiple parts toward the exhaust, meaning I spray the farthest distance from the fan, moving toward it and tacking my pieces as I move forward. Remember, unlike solvent based, waterborne does not dissolve itself.
If a finish is to be rubbed out, with micro-mesh or other means, if you go through one coat into the next you can get ‘ghosting’. My solution to this is what I call a double or back-to-back final coat, meaning I typically will spray a light first coat, just to get finish on the surface. As soon as it tacks, I apply a full wet coat. Once dry, I do my mid-coat scuff sand with 320 grit, then apply a full second coat and again a more aggressive mid-coat sanding. Then I apply another full wet coat and as soon as it tacks I apply another full wet coat
While one coat of waterborne will not dissolve the other, it does have enough burn-in to allow for a chemical adhesion. So the objective here is to catch one coat before it is fully set and apply the second, they then bond together to form one heavy film which has yet to give me a ghosting issue.
On the topic of waterborne and rubbing, having overcome the ghosting issue, I personally find the non-dissolving properties are a blessing. I have no ‘shrink-back’ meaning when I do a mid-coat sanding to a glass smooth surface, the next coat isn’t going to soften it or disturb the surface. After a day or two cure I can sand and rub, wet or dry. Six months or even two years later, the piece I sent out the door was the same.
The heavier solids allow for better grain filling properties and it doesn’t shrink back, so for me the ‘what you see is what you get’ is a super plus.
One last thing, waterborne finishes often lack that warm butter feel solvents offer. It is because there is always a slight amount of overspray. A quick rub with a paper bag or some brown craft paper and it’s like glass.
You will also find chemical and wear resistance is far better.