# HVLP Gravity Cup Spray Gun - Bubbles in cup



## Johndrew (Dec 16, 2012)

I have a HVLP Gravity cup spray gun. I was using a 1.4 tip and put lacquer thinner in it to insure it was clean. I put the lid on and it blew it off. I have huge bubbles in the cup holding the lacquer. It also sputters at the tip when the lacquer comes out.


I took it apart and cleaned it with the same resuts
I put on a new 2.o tip and it does the same thing but smaller bubbles.

What areas should I look at?

Using a 5 hp compresser with 50 ft hose and can get 100 psi at the gun

I have it set at 40


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## Jmayspaint (Mar 26, 2013)

Did you shake the lacquer? Sounds like some air has been introduced into the material.


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## 1963 Sovereign (Dec 14, 2011)

40 psi for a hvlp? is that a bit high?


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## journeymanPainter (Feb 26, 2014)

HVLP requires 10psi, anything more is no longer giving you the qualities of the HVLP, the 100psi is more than a conventional system too. Let the lacquer sit for the day, STIR the next morning, thin accordingly (no more than 10%)


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## CApainter (Jun 29, 2007)

Johndrew said:


> I have a HVLP Gravity cup spray gun. I was using a 1.4 tip and put lacquer thinner in it to insure it was clean. I put the lid on and it blew it off. I have huge bubbles in the cup holding the lacquer. It also sputters at the tip when the lacquer comes out.
> 
> 
> I took it apart and cleaned it with the same resuts
> ...


Check the ear ports on your air cap. From what you described as "the lid blowing off", I would suspect air shooting back into the cup.


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## jason123 (Jul 2, 2009)

sounds like back washing gone wrong. Isnt there a little air hole at the top of the cup that is supposed to be cleared as well.


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## straight_lines (Oct 17, 2007)

Yup that lid cap air hole has to be clear.


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## CApainter (Jun 29, 2007)

straight_lines said:


> Yup that lid cap air hole has to be clear.


Yes. Very important to have that lid hole open to allow paint to flow correctly out of the fluid nozzle. Even the anti spill caps supplied with those gravity fed cup lids will have a small hole in them.


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## Johndrew (Dec 16, 2012)

I have been using it without the lid. Does the Lid have to be on the gun?

I cleaned it with Acetone overnight and it improved the large bubbles being generated and now they are small.

I held my hand over the tip and it generated the large bubbles. I did this several times thinking it would help to backfush some acetone back into the cap and loosed some loose material.


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## CApainter (Jun 29, 2007)

You can spray a gravity fed gun without the lid. Just don't tip it over. By placing your hand over the air cap, you are essentially just blowing air back up the fluid funnel to the cup. 

I would suggest removing the air cap and soaking it in a solvent cleaner, and then use a thin wire to clean the air ports, followed by an air blowdown. I use welding torch cleaners for my air caps, and for the nooks and crannies in the fluid nozzle.


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## Johndrew (Dec 16, 2012)

Cleaned it again and the problem went away. I cleaned it with a small amount lacquer thinner before refilling. Thanks guys.


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## Nabinko (Aug 3, 2015)

Johndrew said:


> I have a HVLP Gravity cup spray gun. I was using a 1.4 tip and put lacquer thinner in it to insure it was clean. I put the lid on and it blew it off. I have huge bubbles in the cup holding the lacquer. It also sputters at the tip when the lacquer comes out.
> 
> 
> I took it apart and cleaned it with the same resuts
> ...


I see you've already resolved this issue for now but here are the 2 reasons a gravity feed gun will "blow back"

1. Air Blockage. If any of your air passages are clogged, the air pressure needs to go somewhere and the easiest place for it to go is straight up through your cup. Thoroughly clean your air cap and fluid nozzle.

2. Fluid nozzle air leak/not sealing .

- On higher end guns, replace the teflon o-ring on your fluid nozzle. (You can get away with the same size standard Buna-N o-ring for a while) 
- If it is cheaper gun without a seal on the fluid nozzle, wrap it with some teflon tape. I found that Yellow Gas line PTFE works the best. (Since it's such a small area, if you find it difficult getting the tape on neatly, once you've cut your length of tape off, slice it in half straight down the middle)


Also, running it at 40 psi sounds right to me. Different guns have different psi ratings at the gauge (going in) to give it the necessary 10 psi or less at the air cap (going out).


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