Trapped Bubbles in Waterborne Finish Coats

Pros troubleshoot another case of those pesky bubbles. July 3, 2008

We are currently spraying Becker Acroma Akvaline 318 water based topcoat using a Kremlin setup, and getting bubbles in the finish. I've added 3% propylene glycol at the advice of my tech but it hasn't seemed to help. There is a lot of air movement in the finish room so it doesn't take long to skim over. I'm fairly new with water based topcoat, so if there are any tricks to help with this, I'd love to hear them!

Forum Responses
(Finishing Forum)
From contributor C:
Does it look like solvent pop - big bubbles 1/32 or larger - or hundreds of small bubbles that give a smoky, hazy appearance? If it's solvent pop, start applying thinner coats (1 mil or less) till you've built up a barrier on the surface, then spray as usual - 2 - 3 thin coats ought to do it (thin, not thinned out). If it's air encapsulation, you'll need to thin and retard to whatever degree is necessary to diffuse the situation. Get with your tech on max amount of retarder allowable per gallon of retarder without compromising the coating's chemistry.

From the original questioner:
Thanks. I have a call in to our tech about thinning the material. I tend to think it's air encapsulation rather than solvent pop, as the bubbles are just too big. I thought about using a smaller fluid tip also - a 06 instead of a 09. I have one on order so we'll see.

From contributor F:
Also, watch the agitation of your topcoat. All water products are highly susceptible to foaming during mixing. After a complete mixing, you only need to keep it moving and not have a vortex. I find most of my water based products don't settle out as hard and stir in better as many of the solvent products did.

From contributor C:
That's air encapsulation, air trying to migrate out of the wood and being captured before it has a chance to come to the surface because the film is drying so fast. Try applying thinner coats, do not change anything but the distance you're spraying at. If that's 6", then try 18" away. If this does not work, you will have to add water or cosolvent such as glycol ethers or butyl cellosolve or other or maybe both. Your chemist will have to determine that. But again, try lighter coats from further away to see if this brings quick relief.

From contributor C:
Forgot - as in original post, further away and lighter coats! A mil or less, slowly build the finish, do 2 or 3 times, let set 15 minutes, and then apply a full coat. You want the wood on the first few coats to be able to breathe a bit. Once this is accomplished it will be sealed enough for a heavier coat to be applied without concern of air entrapment. Mist coats, not wet coats, even if it means being a couple of feet away from the surface.

From contributor W:
Do not add anything to your finish until we figure out your pump and gun settings. What Kremlin pump are you using? 1040 or larger? What is your gun setup? Tip/fan size? What are your fluid and air pressure settings with the trigger pulled?

From contributor L:
I have had the same problem lately. I thought it was the dye stain, Mohawk, but I am not sure. It has only occurred during this winter. I let the stain dry longer and it seemed to work. No dry bubbles. The topcoat is Target lacquer.

From contributor P:
It's waterborne, so it's probably not solvent pop. There's a problem known as shearing, which causes microbubbles when using a pump to spray waterbornes. Do a search on shearing in the Knowledge Base here.

When I bought my pump, my sales rep talked me out of the Kremlin and into the CA Technologies unit, which has a different design to deal with this issue. I spray nothing but waterborne, and I don't have this problem.

That said, I believe I heard that Kremlin offers a different line of tips that might help with waterbornes. I think you can also use a CAT tip on the Kremlin gun. An 06 tip sounds way too small. Try an 11. Waterbornes are usually thicker than solvent based, so you want a bigger nozzle.

From contributor T:
Look at temperatures and viscosities of the material, environment, and the product. A great trick you may try is to heat the WB material. Spraying it at 90 to 100F will reduce the viscosity. Heat, much like when you put oil on your stove, will thin the material. Only by thinning with heat, your solids remain the same, unlike thinning with solvent (water or glycol in this case). To do this inexpensively put a quart can of the material in 1 gallon can half full of hot water. Stir the coating until it becomes warm from being surrounded by the warm fluid outside the smaller can. If it works, you can look at inline fluid heaters or paint pail heaters (I prefer the inline heaters).

Next be sure that the substrate is not too cold or hot prior to coating. For instance, do not bring the wood in from an unheated room and shoot it without acclimation to temperatures recommended. Most WB finishes do not like cold environments. Is your spray area heated?

I also agree... check your agitation rate. Too much will foam the material.

Sheering, as referred to in the post above, is caused by the inability of a thick (sheer sensitive) material to fill the cavity or area of the cylinder of the pump when the piston is extracted. This can shear the material and cavitate in air. This is really pretty rare. An easy test for this is to use a pump with a larger piston and see if it still happens. If you have a 10-14, try a 20-25. Your Kremlin rep would be happy to test and prove your pump system with the material.

Lastly, use an 06-092 or 09-092 series tip. Use about 400 to 600 PSI of fluid and about 12 to 18 PSI of air (triggered). The double atomization series of tips will create better particle breakup on thicker viscosities. These tips were originally designed for stubborn 100% solid WB UV's. Then it was found that they worked great with almost all fluids. Only spray on 2 to 3 wet mils... Go easy, don't spray too heavy. Put a mist coat on and let it dry... Waterbase can look a little ugly while wet, unlike lacquers, but will dry out of it. Don't immediately put hot air on it to force dry, keep it drying at about 70 or 75 for 20 minutes or so.

From contributor C:
Or you might want to try just putting on light mist coats until you seal in the problem.

From contributor J:
Excessive airflow can also be a factor. Can you shut it off or decrease it?

From contributor R:
Get a 09-112 tip. Set the fluid at 40-45 as contributor T says, and the air at 15. Do not hold the gun more than 8-9", so you have to move fast to get a 3 mil coat. I've sprayed a lot of Becker and you can't mist it on, it is too thick. I've never thinned it. One thing I have found a big help is to seal with Zinsser Sealcoat - no grain raising, very little sanding, and waterbornes seem to flow out better on it.

From contributor B:

If you are using a Kremlin 10-14, set your pump pressure at 30 (which converts to 300) and gun air at 30. You also need to retard the finish with a mixture of 1 to 1.5 butyl carbitol and water (1 part bc, 1.5 parts water). Glycol ether DB and propylene glycol works too. Mix 1 pint to 1 quart of retarder solution into 5 gallons of finish. I recommend tip 09-092.

From contributor C:
Maybe "mist" is incorrect - I mean to spray it far enough away from the surface so that a sprinkling of material is deposited. Is "sprinkle" acceptable? Or should I use other terminology, such as extremely light coat?

From contributor R:
I defer to your expertise, but maybe you meant 'tinkle'? :-) I've not been able to lay on Becker thinner than about 3 mils and get it to flow out. But with a good seal coat, it will flow at 3 and build nicely with little sanding.

From contributor A:
Generally, waterborne finishes like to be sprayed in thinner wet coats than solvent-based finishes. It's quite common for finishers to have problems like you describe when they switch to waterborne finishes because they are spraying the finish on too thickly, and/or they are not allowing sufficient drying time between coats. Although flow out enhancers and thinning may help with the problem you describe, if I were you, the first thing I would try is applying the finish in thinner coats - no more than 2-3 wet mils/coat, allowing 1 hour between coats, and applying no more than 3 coats/day. If this doesn't solve the problem, try adding a flow out enhancer/retarder. Be cautious about thinning a waterborne finish with water; you can actually exacerbate the problem by adding water because of the high surface tension of water. I generally thin waterborne finishes no more than 5 - 10% with water.

From contributor M:
In order to get the surface tension of WB finishes down below the critical surface tension of what you want to finish, the formulators must add a surfactant. Trouble is a surfactant is very much like a soap - it wets everything very nicely but it loves to foam. To get rid of the tendency to foam (as much as possible) they also add a defoaming agent. The surfactant is like the soap in your dish pan: in order to get it to foam you must create some turbulence. Then if you wash some greasy pans, the bubbles will disappear (the grease is a defoaming agent).

Now if you look through the posts on bubbles, foaming, air injection, shear forces and cavitation, etc, I think you will notice that this problem is common in Kremlin and you will often find someone saying, "when I switched to my (turbine, Binks/pressure pot, CAT) the problem went away." Do you know of a gun that has more turbulence at the nozzle the Airmix? There are all kinds of theories about what must be happening at the pump to inject this air, but most of the people that have fixed the foaming problem have done so by switching to a 112 series fluid nozzle in their Kremlin (nothing wrong at the pump).

It's my understanding that isocyanates in WB's can react to create CO, and if they do, the result is solvent pop. So either foaming or solvent pop could be the problem. Foaming will show up immediately when the film is laid down. Solvent pop will show up after a few minutes and usually results in very small bubbles and craters. This looks and acts like foaming to me.

What to do?

1. Get a 112 series fluid tip in a size that works for you.

2. Minimize your air pressures to minimize turbulence. You don't want fluid coming out too fast or too much air mixed in. Just enough atomizing air to get rid of the tails.

3. Choke that finishing room airflow. It's a problem for both foaming and solvent pop.

4. If you still have problems you can add a retarder if the problem is solvent pop, but I wouldn't expect it to do much good if the problem is foaming. Soap bubbles like to be soap bubbles and will take a long time to pop by themselves. Better to add a defoaming agent which most WB manufacturers carry. I don't know about BA.

I shouldn't have to say it, but if you're going to write those good pressures down in your procedures book, you also need to make a note of the finish viscosity and make sure that you always have that viscosity in your materials bucket.

From contributor C:
Although I agree with the chemistry in your post, I have found that adjustment of my pu2020 with .09 tip that does this once in a while in cold weather or other weather circumstances can be overcome with no more than applying light coats, by turning the fluid pressure down and the air pressure up. Along with slowing down the air movement in the booth, I think that's all that will be necessary. As to misting, sprinkling, tinkling or whatever - I do mean to get far enough away from the surface to apply a non-wet coat that looks like a non-flowed smooth, somewhat pebbly coating. Once dry, this will ensure you don't get air out of the wood if done a few times. Then after some drying you can apply a full coat. If at that point you're still getting the problem, I defer to the others.

Also, viscosity does have to be correct.