Fluid tips and air caps

[chevynut]

I am confused about fluid tip and air cap selection for paint guns. Maybe someone can help.

I have a couple of guns, a Devilbiss Finishline 2 or 3 with 1.5, 1.8, and 2.2 tips. I only have one air cap for it. I use the larger tip for Slicksand and the smaller ones for stuff like K36 primer.

I bought a Sharpe Platinum gun years ago but I don't know for sure what tip(s) I have but I think it's a 1.4mm. I've never used it since I have yet to do any topcoating.

Anyhow, I am going to be spraying some Cerakote clear and the instructions say to use a .8mm tip because the stuff is really thin. Some say it goes on like water.

Every spray gun I have ever seen has a fluid control knob on it. So why can't I use a 1.4 mm tip and turn the flow knob down? I was thinking the larger tips just allowed more fluid so that's why you used them.

Also, what is the purpose of different air caps, when you can just adjust the air pressure?

[MP&C]

Is that the stuff for clearing the polished aluminum?

[chevynut]

Yes, Cerakote MC-160 High Gloss Ceramic Clear is what I'm going to use on my polished suspension parts.

https://www.clearcoating.com/gallery/detail/6209/Cerakote-MC156-High-Gloss-Ceramic-Clear/

Here's the application guide:

https://www.clearcoating.com/resources/downloads/application-guide-mc-160.pdf

[MP&C]

The fluid control will allow for some "adjustment" but they also have an optimal range. At some point when you get too far out of that range the gun becomes less effective at atomizing or other issues where you'll start to see a degraded spray pattern. Trying to dial something down that much IMO is asking for trouble.. The .8 and 1.0 are typically seen in touch up guns. How much do you need to spray at once? I'd suggest finding a touch up gun...

[chevynut]

I ordered a quart of material to try it but I don't know how far that will go. Maybe I'll just pick up a HF touchup gun and try that. They look pretty good, they're inexpensive, and might work okay for this.

http://www.harborfreight.com/hvlp-detail-spray-gun-46719.html

[chevynut]

So I guess I understand the fliud tip thing to some extent, but I really don't see a practical difference between a 1.5 and a 1.8. I'm sure I can turn down a 1.8 to spray the same as a 1.5. What do different air caps do?

[Rick_L]

Maybe a gross exaggeration will help make the point. Let's say you have a fire hose for a spray gun. You control the amount of paint with the fluid needle. The mixture still has to come out of the big fire hose and it's not going to be uniform, and the material may come out in big "drops".

Different air caps control the flow of air out of the gun. Look at the ones you have. You'll see that the size and number of holes varies. Internal passages in the caps have an effect too.

Just like selecting components for a car or engine, they all need to work together.

The other thing about fluid tips is the amount of solids in what's being sprayed. Slick Sand is probably at the top of the solids list, and usually takes a 2.2 mm tip. Primers usually need 1.8 mm. Topcoats and clears need smaller. Some of this has to do with how much reducer or thinner is used also.

The smaller the tip the better the atomization will be, and the flatter the material will lay if everything else is right. You probably can't color sand this coating, you want it to go on smooth.

[chevynut]

Maybe a gross exaggeration will help make the point. Let's say you have a fire hose for a spray gun. You control the amount of paint with the fluid needle. The mixture still has to come out of the big fire hose and it's not going to be uniform, and the material may come out in big "drops".

That still doesn't make sense to me :confused:. Doesn't the fluid control needle control the amount of fluid that's coming out? My gun has a fluid control AND an air control mechanism. So if the fluid comes in from a fire hose, but I have an orifice at the end of the fire hose of the right size, what difference does it make what size the fire hose is?

Maybe some guns have a fixed fluid orifice and others are adjustable? Or am I all screwed up here as to what the gun controls do?


Different air caps control the flow of air out of the gun. Look at the ones you have. You'll see that the size and number of holes varies. Internal passages in the caps have an effect too.

I guess I knew that ;), but how do I know if I have to change the air cap? What does a smaller or larger air cap do that I can't do with my air control valve? Does a certain size fluid tip have to use a certain size air cap?


The smaller the tip the better the atomization will be, and the flatter the material will lay if everything else is right. You probably can't color sand this coating, you want it to go on smooth.

That makes some sense if the orifice size helps with atomization. But I though the air did that.

[Rick_L]

Most guns don't have multiple air caps available, except for changing tip size. Sometimes there's a different fluid needle to go with a different tip, sometimes not.

Do you realize that the material adjust knob and the tip work together? Look at your gun and think about that. See what's moving when you twist that knob.

Here's another thing to think about. I don't know how much you know about carburetors (reference your recent pompous assed tirade), but you have to change just about everything in a carb to switch it from gasoline to alcohol. Same thing for these different paints/coatings.

[chevynut]

Do you realize that the material adjust knob and the tip work together? Look at your gun and think about that. See what's moving when you twist that knob.

I though the needle travel was restricted when the knob was turned, creating a smaller opening for the fluid.


Here's another thing to think about. I don't know how much you know about carburetors (reference your recent pompous assed tirade),

I don't know why it bothers you that I hate carburetors. Don't take it personally, Rick. ;)


but you have to change just about everything in a carb to switch it from gasoline to alcohol.

That's because a carb set up for gasoline doesn't flow enough alcohol so you have to make everything bigger to get enough fuel to flow through one of those antiquated things. :) :)

So tell me....if you were to run a carb with an adjustable needle inside of a jet, could you run both gas and alcohol with the same carb just by changing the needle setting? I think all a jet change does is increase or decrease flow of fuel. Let's forget about the power valve and float needle, because they have nothing to do with the discussion.

I guess I'm having a hard time seeing how turning the needle in to restrict the fluid flow is any different than using a smaller tip to do the same thing. Why isn't it the same thing?

[Rick_L]

So tell me....if you were to run a carb with an adjustable needle inside of a jet, could you run both gas and alcohol with the same carb just by changing the needle setting? I think all a jet change does is increase or decrease flow of fuel.

It will run, just the spray gun will spray, but in the carb you would need to modify the emulsion tubes too. Air is circulated through them and that affects the fuel flow. The air cap on the spray gun is the equivalent.

The key to any of this stuff is the combination or "system".

Another point on the spray gun, which may not matter too much on the coating job. When you restrict the amount of fluid with the fluid adjustment, part of what you do is limit the output of the gun. With the correct size tip, you spray much faster. This may not be a concern on the coating, but a touch up gun will have that small tip and will spray a bit less too. It will end up easier to do a nice job - and I know that's what you want. I don't know how bad those HF touchup guns are though. Obviously a name brand will theoretically be better. I have one of those HF guns, don't use it much but I have, and it does pretty good in these rookie hands.

[MP&C]

At over $100 per quart, I think I'd follow mfr instructions. If you think you can choke down your 1.4, rock on, let us know how it works out.

[chevynut]

Robert, that's why I'm asking the questions.;) I'm trying to understand the differences. I have a hard time seeing the difference between a .8mm orifice, and a 1.4mm orifice that's decreased to .8mm with the fluid needle. And neither of you has provided a plausible explanation since both orifices would permit the same fluid flow. The air cap is the same as I would use if I had a .8mm tip.

I can see the need to increase the fluid tip size for thick materials like Slicksand. I just wonder if using that 2.2mm tip turned down to the equivalent of a 1.8mm tip by turning in the needle would perform the same as a 1.8mm tip....maybe not. I guess I don't understand why it wouldn't since the same amount of material would flow. Maybe atomization would be affected.

That said, I will probably get one of those cheap HF touchup guns and give it a try as I said before. I want a small spray pattern with great atomization since I have to put a thin layer on my parts without any runs. I may try to set up my Sharpe gun with the same pattern just to see if I can. ;)

Thanks for the discussion. ;)

[Rick_L]

[QUOTE]Maybe atomization would be affected.[QUOTE]

Ding ding ding, we have a winner.

The only thing I can suggest is that flow through a small hole partially blocked in the center is different than flow through a larger hole blocked a bit more in the center.

Sometimes you just have to accept things for what they are. They wouldn't sell all these different tips if they didn't make a difference.

[Troy]

I would say go to Lowes and buy an $7.99 quart of Rustoleum and do a controlled engineering exercise to see what happens.