Why do my seals keep breaking?

[SoftScience]

Hello. Can anybody tell me why this keeps happening to my brush? I disassembled and cleaned the whole thing right before this paint session. I put in some Tamiya black thinned with MLT. I pull the trigger and get good air flow but no paint.

 

I rock the trigger back and forth a bit. Nothing. 

 

I pull out and reinsert the needle. A little squirt of paint but mostly just air. The paint is bubbling wildly in the cup, so I put on the lid to keep it from splashing everywhere. 

 

A few seconds later a giant gob of paint shoots out from under the trigger all over my hands and on my shirt!

 

 

 

This is not the first time this has happened.

 

Is the seal is not holding up? But why? I clean my brush regularly, and have already replaced the seal once. 

 

This is the second gravity feed brush that seems to have this problem. I have a syphon fed Paasche, and I don't think I ever replaced the seals on it. 

[ckw]

You don't say the make of brush - it could be the material the seals are made from not liking either the MLT or whatever you use to clean your brush. I've had some cheap Chinese airbrushes in the past which did not like my cleaning agent (cellulose thinner)

 

Cheers

 

Colin

[SoftScience]

7 hours ago, ckw said:

You don't say the make of brush - it could be the material the seals are made from not liking either the MLT or whatever you use to clean your brush. I've had some cheap Chinese airbrushes in the past which did not like my cleaning agent (cellulose thinner)

 

Cheers

 

Colin

It is a Tamiya sprayworks II 

[ckw]

I don't know that airbrush or the seals it uses - but if they are rubber, they will not stand up to aggressive cleaners. 

 

Of course another possibility is a blocked nozzle which will cause the bubbling in the paint jar you describe.

 

Cheers

 

Colin

[TimB]

I change the black seals on my H&S airbrush frequently. I use Gunze thinners normally but anything more aggressive really damages them. Look at a used seal versus a new one; if the old one is larger then it is absorbing the thinner and needs replacing. I buy non OEM o-ring seals. They are not quite as long lasting as the H&S ones but a lot cheaper so I can afford to change them every few months.  

[SoftScience]

19 minutes ago, TimB said:

I change the black seals on my H&S airbrush frequently. I use Gunze thinners normally but anything more aggressive really damages them. Look at a used seal versus a new one; if the old one is larger then it is absorbing the thinner and needs replacing. I buy non OEM o-ring seals. They are not quite as long lasting as the H&S ones but a lot cheaper so I can afford to change them every few months.  

where do you find non-product specific o-ring seals? 

[SoftScience]

2 hours ago, ckw said:

 

 

Of course another possibility is a blocked nozzle which will cause the bubbling in the paint jar you describe.

 

 

Its very hard to tell with the 0.2 mm nozzle. There doesn't SEEM to be anything in there. The needle comes out cleanly and completely. And there a nice jet of air coming through. I just don't know. 

[Steve McArthur]

My guess is you've got a few things going wrong. First is a dirty/clogged nozzle or possibly the paint path between the nozzle and the paint cup. If I have to pull a nozzle off, I visually inspect parts with bright lights and magnification that the paint path is spotless before reassembly and then test with a blast of cellulose thinner. Cheap hardware store cellulose thinner for cleaning, save the MLT for thinning.

 

Pulling out and reseating the needle is working a little paint past the clog that spurts out. Pulling the needle back can also let paint leak into the trigger area. Bubbles into the paint cup are from air entering the paint path where it shouldn't. The most common point is the nozzle threads are leaking where it threads onto the airbrush. A little Chapstick or beeswax on the nozzle threads should seal that up. Another option is a cracked nozzle but that often shoes up more as a irregular spray pattern or paint spraying before pulling back on the trigger. Really as soon as you see bubbles in the paint cup, stop what you are doing and figure out the problem.

 

FWIW, A lot of people say a clogged nozzle causes bubbling into the paint cup and that is just utter nonsense. An airbrush works by creating a Venturi effect over the the tip of the nozzle to draw paint out of the nozzle by suction. They don't push paint out like a spray can. The nozzle tip is a low pressure area, lower than air in the paint cup. If the nozzle is clogged it is highly unlikely this low pressure air is doing a 180 turn to blow into the tip past a clog and on into the paint cup.  More likely is inside the airbrush where the nozzle threads onto the body and sits in a high pressure area with air trying to escape following the path of least resistance. A cracked nozzle, if bad enough, also lead into this high pressure area an be a source for bubbles in the paint cup.

 

I've never seen paint blow out the trigger area with any force, but pulling the needle with paint in the cup could be leaking some paint into this area that spatters when you block other exits. For troubleshooting, remember air follows the path of least resistance. My guess is you bubbled paint over the vent hole on the paint cap and blocked that. Air is already getting inside the airbrush and the needle packing isn't supposed to be airtight, just paint tight. Block the cup and the next place pressure can go is to blow through the needle packing and out the trigger area. The needle packing seal can be tightened, SMALL turns. Iwata recommends like 1/12 a turn at a time, i.e.. 5 minutes on a clock face. But it's really not supposed to be an air tight seal, just enough to keep paint from seeping past. To tight and it interferes with the trigger moving the needle or the needle spring is too weak to push it forward.

 

Now what's causing the problems in the first place? Where are you thinning your paint? If you mix it in the paint cup adding paint first can contribute to this problem, by priming the paint path with unthinned paint the airbrush can't spray out. I've airbrushed Tamiya XF with MLT, not often, but its never been a problem. I've used AK Real Color more often that is similar with a 50:50 MLT mix.  The paint may also be clumpy if not thoroughly mixed I add 6mm stainless steel ball bearings to all my paints to help mix them.

 

FYI, I don't have your brush, but do have a Tamiya HG-SF 0.2mm. I've only ever sprayed MRP in it. Going through mine, the body and handle are unique to Tamiya, but every other component comes from Iwata parts bins. The nozzle/nozzle cap are Revolution parts. I upgraded mine with an Eclipse Takumi trigger and preset handle. You mentioned you have the 0.2mm nozzle. This is really a detail airbrush and not great for general use. It's also going to be more finicky about getting the paint mix correct. With everything else being equal it could need more thinner than an 0.3mm airbrush. 

 

Edited June 4 by Steve McArthur
typos

[SoftScience]

Thank you @Steve McArthur for the in depth discussion! 

I disassembled and cleaned the brush again, waxed the nozzle threads and didn't pack the needle as tightly. 

 

I noticed some minor splashing and bubbling.in the cup right when I pushed the plunger down in the forward position, but as I pulled it back, it went away. It got more pronounced later during the paint session. Likely as paint built up inside.

 

Second, there IS a hairline crack in the nozzle. Thanks for mentioning that the Iwata parts are compatible, since I haven't seen any Tamiya replacement parts. Are the 0.3 mm revolution nozzle bits compatible as well, do you think?

 

 

 

A few questions. What's best for cleaning out the inside of the 0.2 mm nozzle. I've been using pieces of wire. But I imagine there may be better tools.

 

Which one is the needle packing seal? Is that the one behind the wall that separates the paint cup section from the plunger section? 

[Steve McArthur]

For really troublesome dried gunk I have a metal reamer (the gold tool in that ad) that I've used, but you have to be very careful. I insert it so the nozzle is still loose and just roll the nozzle along one edge to scrape the inside. You don't want to push the reamer in with any force that might split or deform the nozzle. I really only pull this out on some of the used airbrushes I've gotten off eBay that have god know what baked into them. I got one that I'd swear the previous owner had dunked the front end in latex house paint and let it dry into a solid block after he wiped off the outside. Usually interdental brushes and paper dental points are enough and have less chance of damaging the nozzle.

 

That is if I have to take the brush apart. I only tear down a brush when I have problems. For daily clean up I just spray cellulose thinner through until it blows clear and use an old paint brush to scrub loose any paint I can see in the paint cup. Then pull and wipe down the needle. While the needle is out use a cotton swab dipped in thinner to wipe around the bottom of the paint cavity. I also spray a couple drops of thinner through before I try paint when I first pick up a brush for the day, just to rule out problems before I load paint.

 

Yes the needle packing seal is the seal between the trigger and the paint cavity that the needle passes through. 

[ckw]

12 hours ago, Steve McArthur said:

A lot of people say a clogged nozzle causes bubbling into the paint cup and that is just utter nonsense.

I may be misunderstanding you. but covering the nozzle with paper towel or whatever (i.e. blocking the nozzle) will cause bubbles in the aircup with every airbrush I've used. Standard cleaning technique. Yes what you say about low pressure etc. makes perfect sense but if air at pressure cannot get out the nozzle, where else is it going to go other than out the paint cup?

 

Cheers

 

Colin

[ckw]

On 03/06/2025 at 03:33, SoftScience said:

I pull out and reinsert the needle. A little squirt of paint but mostly just air. The paint is bubbling wildly in the cup, so I put on the lid to keep it from splashing everywhere. 

 

A few seconds later a giant gob of paint shoots out from under the trigger all over my hands and on my shirt!

 Thinking about this a bit more, and I'm more convinced its a nozzle problem. First the air was backing up into the cup, then if you closed that off, the air has no where to go except back up through the trigger (though this would suggest the packing seals need replacing).

 

Cheers

 

Colin

[Hunker]

There's a couple of things I can add through personal experience.

A split nozzle will cause the paint you lay down to splatter not bubble in the cup. You won't be able to lay down a fine line, instead you will get a coarse line of paint splatter.

A clogged nozzle won't cause air to bubble in the cup. Just yesterday I put a mixture of Revell 68 and my own thinner into my .02 Sparmax. For some reason - probably a tiny piece of dried paint from the notorious Revell box containers - I pulled the trigger on the airbrush and only air came out. No paint at all and no bubbling at all in the cup. Solution was empty the cup, wipe it out, pour in homemade cleaner and backflush, empty this out, pull the needle back about 3/8" and lock, spray cleaner through at high pressure a couple of times then back came the paint and it worked fine after that.

For the needle packing, push the needle through from the back and you only need to feel a slight resistance, just so you can feel the needle actually going through and that's enough. I've never had paint in the trigger area.

Edited June 4 by Hunker

[cmatthewbacon]

I've only come across on thing that causes bubbling in the paint cup, and it holds true on both my Iwata and my H&S Evolution. That's if the nozzle/cap combo is not tightened fully onto the body of the airbrush. Problems with the actual nozzle (cracks, slits, crud) tend to manifest as weird painting behaviour like uneven patterns, splatter or spraying off-axis. I'd look to the seal of the head assembly to the body, and at the airways through the cap around the nozzle and make sure they are clear.

best,

M.

[Steve McArthur]

6 hours ago, ckw said:

I may be misunderstanding you. but covering the nozzle with paper towel or whatever (i.e. blocking the nozzle) will cause bubbles in the aircup with every airbrush I've used. Standard cleaning technique. Yes what you say about low pressure etc. makes perfect sense but if air at pressure cannot get out the nozzle, where else is it going to go other than out the paint cup?

When you intentionally backflow your brush by blocking off the entire front end, you have changed the conditions. You are blocking the air path and the paint path. The air has no escape but to flow back up the nozzle into the paint cup. The OP is not doing that, he gets bubbling without blocking the front of the brush. In the case of a clogged nozzle, the paint isn't flowing out for the air to ever interact with it. Air should just vent out the front of the brush, no paint and no bubbles in the paint cup, again path of least resistance. This isn't like a spray can, the air isn't pushing paint out. It's sucking paint off the nozzle. The air and paint don't meet until they are outside the airbrush.

 

This is an Iwata type airbrush but all airbrushes are similar. The nozzle tip protrudes slightly proud of the nozzle cap.  If it didn't the air would backflow into the nozzle. Paint is around the needle inside the nozzle, air is venting through the gap around nozzle. You can see where the blue paint is being picked up by the air flow.

 

For air to bubble into the cup something else has to be wrong.  A long enough crack that leads back into the nozzle cap could cause this, but like others mentioned a crack is more likely to cause spatter and other paint pattern problems.

 

The most likely spot for a problem is where the nozzle screws into the the nozzle post. Iwata airbrushes rely on the metal-to-metal mating surfaces at this joint to be precise enough to be airtight. Wear and tear (overzealous cleaning) can lead to imperfections, so a little beeswax on the nozzle threads tend to seal this up. When Iwata ships new brushes they have a little red sealant on these threads to start with, but don't sell it or even describe what it is. Instead they have there own branded thread sealer that is just beeswax in a Chapstick like tube I use it every time I reinstall a nozzle whether it's been leaking or not.

 

This photo is a cutaway of an old style Badger nozzle, but all airbrushes with threaded nozzles are similar. The nozzle threads into a post that the air flows around as it exits the nozzle cap (air cap in Badger terminology). This also shows why the nozzle needs to protrude slightly from the nozzle cap (or air cap) This is the point of least pressure so best vacuum to draw off paint.

 

 

 

To be clear this is a cutaway showing the separation of paint and air paths. I think this is based on an Olympus airbrush with a detachable head like a Custom Micron or the Creos PS-270/289.  The points of failure where air could enter the paint path and cause bubbling are any point where there is a joint in the paint path with air surrounding it. So that's where the nozzle screws onto the nozzle post and in this case where the center of the head base slots into the body. This style of brush typically uses an O-ring for the air seal at the head base. Badgers with detachable heads and H&S drop in nozzles use a Teflow washer to seal against the body that can be crushed over time leading to the bubbling problem. Backflowing is unlikely to blow by the needle packing seal unless the paint cup is blocked for some reason, path of least resistance again. It's easier to vent to atmosphere in an open cup than flow past that big open area and push through the needle packing. 

 

The threads around the perimeter of the head base or nozzle cap are just keeping air inside the airbrush and if these leak you might not see any effect if it's minor or you might get pulsing in the air flow but have nothing to do with bubbles in the cup unless the nozzle cap just isn't fully seated for the nozzle to protrude like it should.

 

 

 

[TimB]

23 hours ago, SoftScience said:

where do you find non-product specific o-ring seals? 

There are on-line sources of viton o-rings, which are the softer black ones that are messed up by agressive thinners.  The H&S ones are a standard size - I'm away from home so can't give you the exact dimensions but if you measure a new one for your specfic airbrush, then check the viton suppliers on line I would be surprised if if were not a standard size.  Also check the parts list for your airbrush - part numbers can give a clue.  Sorry not to be more precise!

[SoftScience]

Steve, thanks for the explanation of airbrush physics. I wasn't aware of how they work down to that depth.

[SoftScience]

17 hours ago, TimB said:

There are on-line sources of viton o-rings, which are the softer black ones that are messed up by agressive thinners.  

That sounds like precisely what I DO NOT want. I frequently spray lacquer thinners, and if I'm already getting backflow issues, then I don't want to add a melting O-ring to the equation.