Tiny Bubbles

Are tools or techniqes to blame for the bubbles appearing in this ICA poly? October 20, 2004

I'm shooting ICA polyurethane, opaque pigmented over MDF. Final finish hand cut and polished. We've always gotten a few tiny bubbles, but recently more and more appear. They aren't really noticeable until you're rubbing out. I thought it was the water compressor, so I got a new rotary screw with the cooled dryer, but I'm still getting the little bubbles. Any thoughts? Could this be a timing issue? We've tried about everything. We're thinking of trying polyester. After years with ICA poly, I'm still not nailing it

Forum Responses
(Finishing Forum)
From contributor R:
What kind of gun and solvents are you using?

From contributor J:
If you're not seeing pinholes before you rub out the finish (topcoat), that would lead me to question the spray equipment. Is it agitating coating to a point where microbubbles (a lot of them) don't have a chance to escape before coating flashes off? What are the locations - all around, on corners, etc? There is a difference between pinholes from bubbles and water contaminating finish as far as what the final coat looks like before you start to rub the surface out. I think the above response was getting to the point of adding some retarder to extend the open time a little more.

From contributor R:
If you are using AAA, turn your pump pressure down. It could be causing foaming.
Fans and drafts could be causing premature skinning of the surface, trapping the bubbles. Again, fix with a retarder.

From the original questioner:
We're using AOM eco system guns, for no good reason other than that our ICA rep sold them to us for this. Interesting - we've thought of all other possible reasons, but you guys are asking about the gun. I love WOODWEB. The foam is noticed mostly in the topcoat and mostly at edges or places of thicker film.

I'm looking to get into new spray equipment if necessary. Any thoughts on the best solution?

It stands to reason in handcut surfaces that using the 39.00 cheapo guns and throwing them away is actually the most cost-effective. I don't condone the idea, but of the thousands we've spent on systems, they don't seem to work any better. We cut and rub out everything, and just want to get the product on securely.

From contributor P:
Thinner coats will get rid of the tiny bubbles. A little retarder will help as well. Thin the material a little and/or turn the fluid flow/pressure down a little.

When you spray the edges and then hit the flats, you get a double pass at the edges that is thicker than it should be. That explains the pattern of bubbles.

From contributor D:
Do you mean urethane 2K and have you tried the Air Mix Kremlin?

From contributor J:
Have sales reps let you test out the equipment for a good two weeks. Advice from us is going to run the gamut of applications and tastes, but this way you get to put the rubber to the road. Without a visual on the bubbles, I'll give you a W.A.G. and say you have your air and maybe also your fluid turned up way too high.

From contributor M:
I am not familiar with that gun, but I see they are available in several different types - pressure feed, gravity feed and siphon feed. Which gun do you have, what fluid nozzle are you using and if you have a pressure feed, what fluid and air pressure settings are you using?

From contributor Y:
The ECO guns are great guns, so I would look at other options before replacing them. My guess is if you replace the gun and keep all other variables the same, your results will not change.

From contributor C:
You are getting trapped reaction bubbles. When the poly is mixed, the reaction process begins. When you spray the product on your work, you should be able to see these bubbles as they rise to the surface and expire after a few minutes as the reaction calms down. However, if the surface of the poly skins over, these bubbles are trapped. That is why you only really notice them after you start to hand rub. I assume that you wet sand first, which exposes the bubbles. You will need to slow the dry time to prevent this, with retarders or thinner and multiple coats. You may want to look into ICA wet on wet process, although I have yet to try it. My samples are on the way.

From contributor E:
"The foam is noticed mostly in the top coat and mostly at edges or places of thicker film."

It could be your gun, could be you need to retard, but since it's only happening at the edges and places where the film is thicker, that says it is a spray technique issue. Trigger pull and release timing may be off as well as the distance between gun and surface at the start and finish of your stroke. I'm betting it's triggering technique and spray pattern overlap, though. Have someone watch you or set up a video camera to tape your technique.

The highest priced equipment and a chemical engineer on hand to vary your mixture according to a meteorologist's recommendations won't get you a good finish if your technique is off.

From contributor D:
If you are using 2K urethane after mixing, you must leave it set for 20 minutes or you will get bubbles.

I agree with the technique advice above - your finishing speed of hand movement may be too slow on edges.

From contributor K:
I agree with most everything everybody has said, except for contributor D saying to wait 20 minutes. I have never waited 20 minutes or even heard of doing so with 2k urethanes. I have sprayed a lot of ICA urethanes and attended one of their classes in Italy, in which waiting was never mentioned.

From Bob Niemeyer, forum technical advisor:
There are a couple things to look at here. Did this problem come about as the temperature got hotter?

If I remember, the ECO gun is HVLP. You don't need HVLP to lay down the 2K PU. Sometimes it injects too much air into the product.

Also, what thinner are you using? In very hot weather, try two different solvent blends.
First try this - mix up a batch of just reducer that is 50% D1010 and 50% D1015. Thin your product with this and give it a try. Next, make a blend of 75% D1010 and 25% D1024. This blend is used to thin your product. The second blend is for very hot weather and should only be used as a last ditch option, as it dries very slow.

If your booth has a large amount of air flow pulling across the sprayed pieces, it could be causing the surface to skin too fast. Try shutting off the booth as soon as you are done spraying.

Another last ditch item is adding one of two different additives ICA can supply. The first one to try would be the AD3 anti-skinning agent. This product keeps the top surface open until all tail end solvents are out. It is not a retarder. Also, watch your air pressure - use just enough to atomize the product.

From contributor D:
2K urethane is mixed 3 qt to 1 qt to make your gallon. Gemithane, a Wood Finishers Supply product, specifically dictates a 20 minute wait period after stirring. This is so the product cross links thoroughly. Excuse my ignorance, but is ICA poly, polyester resin or 2K urethane?

From Bob Niemeyer, forum technical advisor:
We are talking about a 2k urethane here, not polyester. ICA urethanes will have different catalyzation ratios depending on the product. Also, ICA urethanes do not have a "sweat time" or "induction time". This is more often seen with epoxy based coatings.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor L:
We did about 500 tops in the opaque ICA 2k. Heavy build coats catalyzed per ratio first, then used their dilutents about 5%. After the mix we let it steep (sit for about 15 minutes). We re-stirred the product and used the Kremlin pump/gun. Then every 15 minutes or so we would "cycle" the pump from the pickup buck through the drain tube back into the pickup bucket.

Coats were applied very heavy as it was all flat panel finishing - 4-5 mils wet coat. We used the same 2k ICA primer (white) and the same reducer ratio in that. Spray, sand, spray sand, prime coats over P-board substrate. Build coat pigmented 2k ICA as outlined above 4-5 mil wet. Sand and re-coat. 3rd top coat was thinned with 5% acetone and 5% reducer sprayed 3-4 mils.

Product laid out perfect each time and no issues whatsoever. We were replacing vinyl clad tops with a more durable non-cracking non-peeling finish for a call center.
I think your problem is air bubbles from the reaction in the catalyzation process or can be that it is flashing too quick not allowing off-gassing to occur properly in the cure. By letting the product steep 15 minutes before using will allow reaction to complete before stirring again and adding to your pump. By adding the ICA reducer for the product you are using will retard it slightly to allow off-gassing to occur (or air bubbles from wood pores to work through).

It worked perfectly for us with our big Kremlin pump as outlined above and about 500 tops came and went without an issue.