Spray Painting Lacquer

[ian]

One thought - if I'm using waterborne finish, do I still need to dry the compressed air? I imagine a bit of moisture will not negativly impact the finish, but of course I still need clean air. What do you think?

yes
in part successful spraying is about controlling the viscosity of the finish. With an unknown quantity of water in your air line (both as vapour and as drops) I can't see how you can keep the viscosity constant

Okat also another big question - compressed air or turbine? I see Funji make a 'Mini-Mite' system which in total is a similar price as getting an affordable compressed air setup and looks more convinient and portable. It also comes with the more expensive Fuji T75 gun (not sure how different it is than the 'M' gun). What to do, so many choices?!

you have two basic choices
High Pressure air as you originally asked about, or
High Volume Low Pressure -- which is the turbine type system you're now asking about.

sorry but I can't really help you choose between the two

[Tenson]

you have two basic choices
High Pressure air as you originally asked about, or
High Volume Low Pressure -- which is the turbine type system you're now asking about.

I don't think it's as clear as that. Many people are using HVLP guns run from a compressor. Provided the volume of air can be deleivered at the required pressure it will work - but what are the pros and cons of each?

For example the Iwata W71 gun manual kindly indicates how many CFM are needed for the various nozzle sizes. A 1.3mm nozzle with gravity feed needs about 7CFM. I can get a compressor that is rated at 9.6CFM (what pressure?) so should mean my tank never empites.

[ian]

But it would probably mean that your compressor was running continuously.

Typically small compressors like the one you are referencing are more suited to pumping up tires and operating an air nailer, rather than spray painting.

[Tenson]

What would be a good entry level air compressor for the Apollo A7500GT or Fuji T75 Gravity gun?

Given the size of what I need to spray I doubt I will paint for long streches non-stop.

[rob streeper]

I've only used one turbine pumped HVLP and I didn't like it. The unit I used was made by Titan and it had a 5 stage turbine. The gun was drippy, leaked paint, the compressor was loud, the gun got unpleasantly hot, the gun and hose were heavy, hard to adjust and a lot of work to clean. Sata makes a pressure feed reservoir and regulator that can be added to any of their guns, mine is on the right in this picture.

SATA NR95 collection.jpg

Iwata also makes a pressure fed gun but I can't locate a link for you at the moment.

[ian]

Given the size of what I need to spray I doubt I will paint for long streches non-stop.

Even a medium sized speaker box might take 1 to 2 minutes to spray, and with a small compressor and receiver, it's probable the compressor would cut-in 10 to 15 seconds after you activated the gun.

I can't at the moment find it, but I recall BobL posting some comments about the limitations of small compressors not so long ago.

[Bohdan]

With a reasonable gun it should take no more than 15 secs to paint a speaker box and this would have pauses while you rotate the box.

[Tenson]

I think I have come to a decision. I'll probably go for the Fuji T75 gun with their mini-mite turbine. Yeah I know... after all the questions I want to get an off-the-shelf system!

Here is some of the things I've been thinking about.

1) If dry air is so importiant, why do the turbines (even the rather pricey ones) not have a filter to remove moisture?

I think the answer to this is to do with pressure differential. When a compressed gas expands, moisture is condensed from it. The bigger the pressure difference on expansion, the more likely moisture will consense. So when we have a compresser with air in the tank at a whooping 100PSI or more and we allow it to expand, first at the pressure regulator and then again at the gun nozzle, moisture will certinaly condense if there is any in the air. The large pressure differential from 100PSI to atmosphear is why it becomes super importiant to take out the moisture before-hand. When we talk about an HVLP turbine the air is never compessed more than about 16PSI and so when it expands at the gun nozzle there is far less likelyhood of condensation forming, even if it is held in the air. That I think is why we don't see much effort made to dry the air from HVLP turbine systems. Compressing the air just creates a problem that needs to be fixed.

2) If HVLP guns work at a lower pressure and have the same cap size, how on earth can they use more air volume? You have a cap with say a 1mm hole and the air pressure behind that hole will dictate the air quantity that passes. This was really confusing me.

I think the answer is simple. The cap size is not what restricts the air flow - the gun needle is. So I guess an HVLP gun will have a larger diameter needle than a conventional high pressure gun, even if the cap size is the same. Is that right?

3) Why are HVLP turbines spec'd at something like 100CFM even when the guns never use more than 20CFM?

I guess the answer is that turbine CFM is spec'd at free flow into atmosphear? Is that right? So when a gun forces the turbine to work at a pressure of say 20PSI, the max. air-flow the turbine can provide will reduce significantly, probably to something like 40CFM.

4) Waterborne finishes have solids suspended, not dissolved, so they need more pressure to atomize well. Surely a conventional high pressure gun will work better?

I think this may well be the case. However for myself I just don't want the overspray a high pressure gun gives in my small workshop. The cap size and gun design will also affect the atomization so pressure is not the only consideration.

Okay then, I hope laying out my thoughts might help someone in the future! Please add anything you think I've missed or got wrong.

Simon

[ian]

Hi Simon
a bit more reading for you

Exhausting Overspray in the Home Shop - Popular Woodworking Magazine

HVLP’s evolution is an interesting story | Woodshop News

Spraying Right, Spraying Safe, Where to Use Your HVLP Sprayer @ The Finishing Store News

[Tenson]

Thank you! Something for the evening there.

[homey]

Hi Tenson,

For interest you might like to check YouTube where there are several noise level comparisons between the various Fuji HVLP systems. This may not be an issue for you, but could be worth 5 minutes to view anyway?

cheers,

Brian

[Tenson]

I've heard the Fuji Mini-Mite is rather noisy, but the 'Quiet' versions are a lot more pricey! I suppose I could get the Fuji or Apollo gun and a differnt turbine but it doesn't seem very cost effective and I'm not sure other turbines are better?

For others who read this thread - I've had recommendations for Morrells (a UK brand) and Sayerlack water based lacquers for spraying. Sayerlack especilly seem to offer a lot of range.

[rob streeper]

Simon,

Please let us know how it goes with whatever system you choose. I've looked at the Fuji an Apollo systems too, but my underwhelming experience with the Titan put me off of the idea of a turbine.
One of the things I know you'll find is that the output of the turbines is hot, you can't escape the laws of physics. When you compress a large volume of air in a short period of time you get heat.
To me the water based lacquers are rather like vegetarian bacon - poor imitations of the original. Nonetheless I'm interested to hear of your thoughts of the system and coating you do end up going for.

Cheers,
Rob

[Tenson]

I'm sure I'll be back here when I get the system. I've got to get my workshop first!