Richard Aerni on mon 15 jan 07
On Mon, 15 Jan 2007 19:41:52 -0600, Vince Pitelka wrote:
>
>Debby -
>You can swing the side walls in a little, so that the opening at the front
>is narrower. Or, if you need the full space inside, you can hang some
>shower curtain material on either side in the front, creating a narrower
>opening. That should increase the velocity and volume of the air entering
>the booth, and reduce the amount of overspray that comes back towards you.
>Of course, you should always wear a dust mask as well.
I have a 4 foot by 4 foot front opening in my spray booth. I hang a
fluorescent light from the top of the box for lighting. When I am working
on small pieces, I hang a board on the front of the spray booth that cuts
down the open area and increases the suction from the fan. I have an old
electric pottery wheel mounted so that the wheel comes above the bottom of
the spray box, and so with the foot pedal controlling wheel speed, both
hands are free to work the spray gun and glaze containers, and there is no
need to stick your head into the spray box to turn the banding wheel,
thereby getting right in the thick of the swirling glaze. This is the main
problem I have seen with most spray booths...one must get one's hands in
there to turn the piece. What a sucky thing to have to do and so simply
remedied.
>You will have to do plenty of experimentation with your spray gun. Some
>spray guns work best at 25 lbs, some at 45 lbs. Also, the consistency of
>the glaze makes a huge difference. Be sure to always screen a glaze before
>spraying, and it might help to thin it slightly. Always look for that shiny
>wet spot where the glaze hits the surface of the form being sprayed. It is
>essential that the glaze pool slightly on the surface, and one of the tricks
>of learning to spray effectively is to make sure that the glaze is still wet
>when it hits the surface, but to avoid spraying so heavily that it runs
>excessively.
Funny how different people do different things differently! I spray my
glazes 100% of the time for the past 27 years, and tend to mix them up a bit
thicker than I would for dipping glazes. Of course, I also mainly single
fire, so I don't want lots and lots of water on the pot. But, some of my
glazes for spraying are very very thick and viscous. Works for me...
Best,
Richard Aerni
Rochester, NY
Fred Parker on tue 16 jan 07
Hi Debi:
You didn't mention the type of spray gun you are using. The gun type and
setup are critical to minimizing overspray, so here are a couple of points
to consider:
Based on the pressures you mentioned I assume you are using a conventional
(not true HVLP) gun, or perhaps an HVLP gun of the type that operates on
high air pressures at the connection to the gun. Either way, within your
gun there is a part called a "fluid tip." The fluid tip governs the
amount of liquid the gun can discharge. Some have small orifices designed
for light-bodied fluids such as lacquers while others have larger orifices
suitable for higher-viscosity materials like enamels (and ceramic
glazes). Unfortunately, the literature supplied with many guns is
worthless, so many first-time users of spray equipment have to learn the
details the hard way -- or not at all. Most spray guns can accommodate
different fluid tips, and most guns come supplied with a variety of them.
If yours does/did, try the one with the largest orifice, and reduce the
air pressure. This will result in a coarser spray and MUCH less
overspray. A coarser spray might be a problem with some paints, but with
ceramic glazes it is not.
A second solution is to spend the $$ to purchase a "pressure feed" gun.
These are great for viscous or heavy fluids because the fluid container
itself is pressurized. Fluid flow into the atomizing jet of air at the
air cap is therefore a function of fluid pressure, and NOT atomizing air
pressure. With these guns you can reduce atomizing air pressure down to
zero and still get a stream of fluid from the nozzle. Unfortunately, they
tend to be on the pricy side, but they work very well.
Or, you could get one of those self-contained $80 HVLP guns from Harbor
Freight and forget about needing the compressor. I have used both the
conventional airspray and the HVLP. For lacquers on furniture or high-end
automotive finishes I don't like HVLP. For ceramic glazes, it's a good
affordable solution if you can stand the noise AND if you don't need a mix
of smaller guns like "The Critter" or airbrushes.
Your father is right in telling you to have some source of makeup air when
operating a spray booth. Have windows or doors open. The problem is
usually not so much a "damage" problem as it is a functional one, although
you can burn out a fan motor this way if you do it for a long time. The
exhaust fan is dumping a large volume of air from inside the booth to
outside the room. Its flow is what eliminates the overspray from the air
you breathe. If you seal the room that large volume of air needed for
exhausting is not available and the booth will not work. I doubt the
pressures involved are sufficient to pop your eyes out, but you will get
overspray washback into the room.
By the way, it's still a good idea to wear a P-100 respirator when using a
booth. You won't be able to see all of the overspray, and some of it WILL
escape back into the room where you will breathe it.
Fred Parker
On Mon, 15 Jan 2007 18:38:05 +0000, dwichman@frontiernet.net
wrote:
>
>My questions concerns the air compressor. I began with the pressure
>at 25 - 30 lbs (as close as I could tell on a regulator with
>condensation on it's face). We had a light clipped on the edge of the
>booth so I could easily see any overspray coming out of the spray
>area. It looked as though very little came out of the front of the
>booth and then really promptly was sucked back in. The spray wasn't
>maybe as heavy as I expected at this pressure, so we turned the
>pressure up to about 40 lbs. The spray was heavier and made covering
>the pot quicker, but I noticed the overspray that came out was quite a
>bit more - looked like air turbulence. It curled out much more
>dramatically and then most went back into the booth. Is this normal?
>Is there a way for me to spray at the higher pressure and have better
>control over what comes outside the opening of the booth? I will wear
>a mask regardless, but it would be a quicker job at the higher
>pressure.....
>
>I was also curious about something my father told me. He installed
>commercial skylights before retiring. He said to make sure and have
>plenty of incoming air, because between the fan and compressor you
>could create a damaging vaccuum in an enclosed area. Does this sound
>viable?
>
dwichman@frontiernet.net on tue 16 jan 07
Thanks so much for the pool of knowledge about spray booths. It is =20
very helpful and I will get to try out some of the ideas this weekend. =20
Most of my work is not very large, so I will bring in the sides of =20
the spray booth and also use some heavy vinyl to close up as much of =20
the front opening as is feasible while spraying.
I really like the idea of using a wheel with the foot operated pedal =20
so you have both hands to work with the spray gun - and you don't have =20
to reach in and turn the work by hand. I will be on the lookout for a =20
cheap used electric wheel or instructions on how to make something =20
that may work like that....
The spray gun I purchased to use is the Critter.....maybe it wasn't =20
the best choice. I thought the idea of using pint mason jars as the =20
glaze containers would be very handy, and didn't really think through =20
or research into what may be the best spray gun. It seems it was =20
mentioned several times in the archives as a suitable gun.
So the HVLP, high volume low pressure - I looked on Harbor Freight and =20
it looks as though there is a large range of prices. It also looks as =20
though some of them are gravity fed in addition to being HVLP. =20
Truthfully, the gravity fed ones look as though they may be somewhat =20
fatigueing to my 50 year old hands, so I would prefer not to go with =20
that. According to your experience, are the $40 to $60 dollar HVLP =20
guns good enough for this use? I notice some mention just one =20
nozzle(like .054) size and some don't mention any nozzle size, but =20
have two spraying positions. Do I need to worry about a minimum =20
nozzle size for glaze - I am sieving all. It makes sense that the =20
coarser spray would have less overspray. Does anyone have a =20
recommendation on an HVLP model spray gun?
Thanks again -
Debi Wichman (again realizing the more you learn the more there is to learn)
Cookeville, TN
http://www.elementterra.com
Richard Aerni on wed 17 jan 07
The main reason I can spray thicker glazes is because I use a sand blaster
gun to spray, not an HVLP or paint sprayer. It works much like the critter,
but unlike the critter, you can adjust the flow rate of the glaze as you are
spraying. It has a large (1/8 th inch) diameter opening for the glaze, so
it accepts a thicker stream. What is nice is that I use quart plastic
yogurt containers for my glazes, and with the "straw" to suck them up and
into the gun, I can change glazes in a second or two without losing the
continuity of the glazing process.
As for a wheel for the booth, well, I've six wheels in my studio, acquired
over the years, some for little or no money, and it just seemed easier to
put those to work. I'm sure with a little ingenuity people could figure out
their own solution...that's what I did. No one told me how to build a spray
booth, I just thought about it, tried something, tried something else, kept
what worked and discarded what didn't. In my experience, I've never had any
problem with disrupting the flow of air leading to eddies by narrowing the
opening to the booth itself. Far from it...the draft increases as the
opening is reduced. Of course, that's just in my booth...yours may be
different.
Best,
Richard Aerni
Rochester, NY
Fred Parker on wed 17 jan 07
Hi Debi:
More on spray guns...
I failed to mention what is probably the most important single factor in
using a spray gun: viscosity. You will have to thin your glaze somewhat
before it will spray properly. Symptoms of glaze that is too thick:
excessively high pressures are needed to spray and you will always get a
ton of overspray because the atomizing pressure is so high. I noticed
Richard A. said he gets the result he wants by using thicker glazes
sometimes, and I'm sure he does. However, for a beginner I'd recommend
trying a thinner glaze and a lower air pressure to reduce overspray.
Here's some background as to why: A Critter gun (and many others) "suck"
the fluid out of its container by a process called "venturi action." In a
simplified version, the jet of air blows across a hollow tube (venturi),
and in the process it creates a slight vacuum inside the tube, which in
turn is connected to the fluid reservoir. That sucks the glaze up to the
top of the tube where it mixes with the jet of air and is blown off into
space as an atomized fluid, which is what you direct toward your pots. If
the fluid (glaze) is too thick, you have to jack the air pressure up just
to lift it. Then, when it reaches the air jet, the air is at such a high
velocity (because of the high pressure needed to lift the glaze) that it
blows it everywhere instead of on your pot. This is the overspray.
Thinning the glaze will enable you to lower air pressure and reduce
overspray.
By the way, nothing wrong with the "Critter" gun. Just get the viscosity
right and it will be fine. In fact, because it shoots a fairly narrow
pattern it's well suited for spraying glazes, especially if you plan to do
some blending with other glazes.
Re closing the booth opening with heavy vinyl, I'd be careful about that.
For a spray booth to work properly you need a smooth ("laminar") flow of
air through it from front to back. Think of it like water flowing down a
deep stream. The goal is to move all of the contaminated air AWAY from
you with as little disturbance as possible. As you block the opening you
interfere with that smooth flow resulting in eddys, swirls, areas of
higher velocity etc. which can actually blow overspray back out of the
booth. (The analogy would be the fast, shallow stream with rocks. Your
goal is a smooth, even flow of air away from you and through the booth's
exhaust. MUCH better to get your gun and fluid viscosity adjusted
properly. That should take care of the problem.
Re HVLP and Harbor Freight, the gun I mentioned is a self-contained unit
(compressor and gun together), NOT one of those guns that connect to a
conventional air compressor. Harbor Freight is not too clear about the
difference. Mine cost $90. Its compressor is actually a blower (remember
HVLP means "high volume, LOW pressure" so it doesn't need the 30-40 psi
the other guns require). When running, it sounds like a vacuum cleaner,
in fact, I have seen home vacs with HVLP spray guns as accessories. You
can see it at itemnumber=44677> My HF gun came with three fluid nozzles, although I've
only used one of them.
I don't think there's much of an advantage to a "gravity feed" gun for
glazes. Those guns were developed for spraying fine finishes (automotive
finishes, furniture finishes etc.) that are typically thin viscosities and
need very high atomization, but not so much that the gun's transfer
efficiency is compromised. Somebody came up with the idea that using
gravity to let the thin fluid flow down to the atomizing air by itself
instead of having to jack the atomizing air up to facilitate lifting the
fluid from a bottom reservoir might reduce overspray somewhat and increase
transfer efficiency. For ceramic glazes we typically don't split the
hairs this finely...
Re using a wheel for spraying glaze, that's a great idea if you can afford
it or already have a used wheel! I can't/don't, so I rely on a $30
banding wheel. I have to spin it myself, but now that I think about it I
should be able to spend the $500 I saved and get myself a neat new toy!
Fred Parker
On Tue, 16 Jan 2007 22:40:48 +0000, dwichman@frontiernet.net
wrote:
>Thanks so much for the pool of knowledge about spray booths. It is
>very helpful and I will get to try out some of the ideas this weekend.
> Most of my work is not very large, so I will bring in the sides of
>the spray booth and also use some heavy vinyl to close up as much of
>the front opening as is feasible while spraying.
>
>I really like the idea of using a wheel with the foot operated pedal
>so you have both hands to work with the spray gun - and you don't have
>to reach in and turn the work by hand. I will be on the lookout for a
>cheap used electric wheel or instructions on how to make something
>that may work like that....
>
>The spray gun I purchased to use is the Critter.....maybe it wasn't
>the best choice. I thought the idea of using pint mason jars as the
>glaze containers would be very handy, and didn't really think through
>or research into what may be the best spray gun. It seems it was
>mentioned several times in the archives as a suitable gun.
>
>So the HVLP, high volume low pressure - I looked on Harbor Freight and
>it looks as though there is a large range of prices. It also looks as
>though some of them are gravity fed in addition to being HVLP.
>Truthfully, the gravity fed ones look as though they may be somewhat
>fatigueing to my 50 year old hands, so I would prefer not to go with
>that. According to your experience, are the $40 to $60 dollar HVLP
>guns good enough for this use? I notice some mention just one
>nozzle(like .054) size and some don't mention any nozzle size, but
>have two spraying positions. Do I need to worry about a minimum
>nozzle size for glaze - I am sieving all. It makes sense that the
>coarser spray would have less overspray. Does anyone have a
>recommendation on an HVLP model spray gun?
Brenda Funk on tue 23 jan 07
Subject: Re: Spray Booth Questions
As for a wheel for the booth, well, I've six wheels in my studio, acquired
over the years, some for little or no money, and it just seemed easier to
put those to work. I'm sure with a little ingenuity people could figure out
their own solution...that's what I did.
I plan to set up to spray soon, hopefully this winter. I have an old record
player in the basement that I plan to use as a platform for the pot. I'll
bet 16 RPM's is about right. If not, maybe I can rig up a rheostat to slow
it down.
Brenda
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