I have fought air bubbles in poly for years, and just recently discovered a tremendous solution (for me) which work equally well for you.

Use General Finishes wipe on polyurethane - Arm-R-Seal. It goes on like no poly I've ever used... smooth as silk with absolutely zero bubbles and NO streaking. When using a wipe on method, the applicator has a lot to do with the issues of bubbles and streaking. In my case, I used the same applicators for both teh Minwax and the GF products, and will NEVER use Minwax products again. When wiping, the coats are thin, but the finish is really hard. The GF product can be sprayed, but I've not personally done that yet and cannot speak to it with real knowledge (they, GF, have some videos which addresses how to use their products). I just know that I'm in love with finishing again! Mind you, it's NOT cheap, but I firmly believe that you generally get what you pay for when it comes to material pricing.

Over the years, I have been increasingly disappointed with the Minwax line of products, and am growing increasingly impressed with the General Finishes products.

 

I have heard a lot of good things about General finishes. I've used their products before including the Java Gel Stain. I will give this a try and see how it goes if I can't figure out this HVLP issue. I just bought a turbine system today from Harbor Freight and going to try that. If that fails your option is next. I always dread finishing because of the monotony of it but I've never had a bubble issues like this. Thanks for the input!

 

I have used General Finishes High Performance urethane topcoat with both airless sprayer and Fuji HVLP.

The airless was pumping too much product and/or I was not sufficiently skilled. Most of the can just became overspray, and I had a few problems with runs, but no bubbles.
The Fuji HVLP was the better tool. Still had to be careful to avoid runs on vertical surfaces, but no air bubbles.

 

I imagine having the perfect ratio of air to product is probably very important and also making sure the pressure is set just right. Trying to get that down seems to be a bit of a trick. I'm wondering if possibly there was water in the air compressor tank that then condensated and got shot through the gun. Either that or maybe my pressure was too high? I have this three stage turbine HVLP sprayer now and hoping that is easier.

 

After numerous attempts at trying to brush the polyurethane Finish without bubbles I decided to use an HVLP spray gun to put on the final coat. I had everything sanded down nice and smooth. Built a relatively dust free spray booth just to perform this task. After I initially sprayed it it looked beautiful. When I returned 2 hours into the dry time I found all of these little bumps, most of them the size of a pinhead that you could only see from an angle, all over the finish. And it's not dust. It literally looks like hundreds and hundreds of tiny little bubbles. I'm ready to give up and paint the door. Does anybody have any ideas on what could be causing this?

The only other input that I can give you is I'm using a husky 40 PSI gravity feed HVLP gun. 20 gallon compressor. I set the tool pressure to 40 PSI and then I have a regulator at the bottom of the gun. When I triggered the gun on the test piece I noticed that the pressure dropped considerably. I had to crank the air pressure on the compressor up to around 75 PSI in order to maintain good output. It was around 50 psi at the gun. I don't know if that has anything to do with it because it was the only way I could get good output. And this is very similar to the defects I was getting when the wipe on poly was drying. It's been a week since my last finish and 24 hours since sanding. Used 10% VM&P Naptha in order to thin the finish.

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Just a thought, but is Naptha all you can thin the product with? .... If I recall correct, Naptha has a pretty high flash point, which to me equates to rapid evaporation. If there is another thinning option with a lower flash point (doesn't dry so fast), you may want to give it a try. .... The faster the vehicle evaporates, I would expect it more likely to trap air, especially when sprayed. ... I always felt the best solvent based enamel finishes were attained by slow drying, which is more likely to let air escape AND level itself.

Also, did you wipe with a tac cloth before application? If not, could still be some sort of contamination on the surface.

 

Just a thought, but is Naptha all you can thin the product with? .... If I recall correct, Naptha has a pretty high flash point, which to me equates to rapid evaporation. If there is another thinning option with a lower flash point (doesn't dry so fast), you may want to give it a try. .... The faster the vehicle evaporates, I would expect it more likely to trap air, especially when sprayed. ... I always felt the best solvent based enamel finishes were attained by slow drying, which is more likely to let air escape AND level itself.

Also, did you wipe with a tac cloth before application? If not, could still be some sort of contamination on the surface.

You might have a really good point with the naptha. I started using that because a guy that sprays doors out in California said he uses that product and likes it better than mineral spirits. But I do have mineral spirits which is slower drying. I will try that on tomorrow's finish. I don't like using a tack cloth because I have found that sometimes it leaves a sticky residue that creates more issues. So I always do a wipe down with a mineral spirit dampened cloth and then follow it up after that dries with a vacuum pass over it.

 

What kind of filtration do you have on your compressor lines?

 

Unfortunately none. Never knew about that.

 

How often do you drain the water out of your compressor tank?
When I lived in humid fla I had to drain off the water often along with using a water separator.

 

Honestly I hadn't thought of that. Haven't used this compressor in about 5 years or so. Possibly more. And didn't drain it before I used it. Really dumb of me. And wasn't even familiar with what a water separator is.

 

I also have a 3-stage Fuji HVLP and have used it to spray BIN, GF milk paint, and Benjamin Moore Advance for kitchen cabinets. Getting the right nozzle for each medium and balancing the flow accordingly can certainly be tricky. When I had to thin the milk paint, I used DI water to get a workable viscosity. I also found that had to make noticeable adjustments (different nozzles and flow needle adjustments) between those three materials when I did two sets of cabinets several years ago. One thing which really helped me was using a huge refrigerator box to test my spray patterns prior to actually laying product down on the cabinet doors. [ I also had nearly 60 cabinet doors and drawer fronts to do, and used a home-built drying rack system which would hold all of the doors and fronts. Since I did them one-by-one on a turntable system in a booth, it was tedious and time-consuming, and I had to stop occasionally to dismantle and clean up the gun/nozzle to maintain a good pattern over the 4-6 hours it took to do the job. ]

 

Bare minimum is a disposable plastic air filter you screw right onto your air line. Theyre like $5. Better to couple that with a glass bubble filter coming out of the tank. and yes, drain your tank before every paint job.

 

It's good practice to drain off the water from the tank after each use. Besides keeping water out of whatever air tool you use it also goes a long ways toward preventing rust in the tank.