Today's Message Index:
----------------------
1. 08:26 AM - Painting web Site - tips, educational data & low cost internet painting (P M Condon)
2. 02:35 PM - fuel injection (Jim Jensen)
Message 1
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| Subject: | Painting web Site - tips, educational data & low cost internet |
painting
supplies
--> Rocket-List message posted by: P M Condon <pcondon@mitre.org>
Long post
www.autobodystore.com
A great site to learn (or relearn) painting basics and purchase
professional supplies. I purchased my Devilbiss Plus Gravity gun from
them at 150$ off list. The paint gun was selected based on conversations
with Len Stewart, owner/operator of the online shop. Len will answer
questions and generally help you with your paint needs. I found the
counter folks at the jobber stores less then helpfull, lacking in
product familarity and generally lacking in the painting knowledge in
general. I only buy the paint from them now and rely on the data I got
from the forums.
Here is the section on "Painting Basics" from Len's web site......
Being HVLP and low VOC products are the way the industrys going I will
be referring to them in this discussion on painting and paint guns. Most
all basic issues dealing with HVLP can be applied to conventional guns,
atomization is atomization. The HVLP just arrives at it differently.
The object of the spray gun is to break up the primer/sealer/paint/clear
(I will call this PSPC from here out) into small particles and lay
them in neat little rows on the panel being PSPRed. So the whole outcome
rests on how well the gun is doing this. Picture the droplets of PSPC
coming out of the fluid tip of the gun and then the air slapping them
into smaller droplets.
You have two things that help you with this process, air and solvent.
Solvent can mean something that is already in the PSPR from the
manufacture or something the manufacture has told you to add to it. By
the way, you should always mix in proper ratios as instructed in the
tech sheet. The thinner (less viscosity) you get the PSPR or the more
air you have at the fluid tip of the gun the more it will break up the
PSPR. The target for you is getting the perfect balance needed. Too much
solvent and the PSPR will have no body, fill, durability, etc. Too much
air and you blow the PSPR everywhere but the car, poor adhesion,
excessive texture, etc.
So, the answer is proper air supply and gun (and fluid tip) choice and
how you adjust it.
With todays high solids-low VOC (Volatile Organic Compound, you know
the bad stuff that goes up into the air we breathe) products there is
less solvent. And with HVLP guns there is less air at the cap to break
up the PSPC, proper air supply and gun setup is more important than
ever.
FIRST THINGS FIRST, your compressor and air supply.
An HVLP gun requires more VOLUME of air to operate (the V in HVLP, High
Volume Low Pressure). Now you may notice that your HVLP gun is adjusted
at maybe the same PSI as an old conventional gun, around 50 lbs at the
gun (many HVLP guns are set at much lower though) so where is the Low
in PSI they are talking about? It is at the actual air cap where the air
and paint come out. An HVLP gun has only 10 lbs at the cap while a
conventional has upwards of 50! So the VOLUME of air (CFM, Cubic Feet
per Minute) is the key to proper atomization with an HVLP.
If you have a gun that requires 15 CFM you will need a compressor and
plumbing that will produce that at a very minimum. There are HVLP guns
that need as little as 7.5 CFM so you can get good results even from a
smaller compressor. Air supply is a complete subject by its self so
lets assume that you have the air supply needed and move on to gun set
up.
So atomization is the key, but why? Why cant you just lay it out wet
and let it flow, as an old painter will say. Picture a jar full of
bbs, they will represent well small, atomized droplets of PSPC. The
gaps in between the bbs is solvent. Now picture a jar filled with
marbles, they will represent large, poorly atomized droplets of PSPC.
The gaps in between are, you guessed it, solvent.
If you apply your PSPC in large poorly atomized droplets, what you will
have is a film full of solvent. This can and will cause slow curing,
shrinkage and dieback (the loss of gloss in the hours and days after
application).
So, now that we have learned the need for gun set up, how do we do it?
Lets start with the fluid tip choice. The newer high solids low VOC PSPC
products need to be broken up more, so a smaller fluid tip is needed.
Basically you want the smallest fluid tip that will still allow you to
PSPC the particular part you are PSPCing keeping the entire thing wet
and in a fair amount of time. In other words a 1.0 tip would be
beautiful for clearing one fender, but would be lousy to paint a
complete. The application would be way to slow and the first panel would
be way to flashed by the time you got around back to it. So you need to
compromise, a 1.3 is a great all around tip, while a 1.5 though getting
a little big, can get you by. If you read the tech sheet on the
particular product you are shooting, it will have a recommendation for
fluid tip size.
There are needs for other tips, for instance when shooting polyester
primer you may need as big as a 2.3, but for urethanes and epoxies, the
1.3 or 1.4 will work great. If you plan on using a pressure pot or paint
a bus, all bets are off and we would need to study a little bit more.
As an example of the use of a 1.3 tip I did a test once that proved the
point well. I shot two panels of metal with a med solids urethane
primer. One was shot with a 1.3 super high atomizing top of the line
topcoat gun. The other was shot with a 1.5 (or a 1.7 I cant remember)
hoser primer gun. Three coats were applied and after a full cure (the
one shot with the larger gun took MUCH longer to flash and cure by the
way) the film thickness was measured. The one shot with the 1.3 tip was
2 tenths of a MIL thicker! The larger gun laid out the marble sized
droplets full of solvent and when the solvent flashed the film shrank.
Air supply is a subject that could fill many pages by its self. So we
are going to assume you have that covered and move on to gun set up.
You need to tune your gun EVERY TIME you use it just as you would tune
a guitar before you perform. This is done with a very basic spray out
pattern test. This very basic test tells you how your gun is atomizing
and you adjust it to achieve the best atomization you can.
Lets do a spray pattern test:
Set the fan width as need (you dont want to change it after you have
tuned the gun). Turn out the material knob about 2 turns. This is
the mixture adjustment, kind of like the idle screw on a carburetor.
The farther in it is screwed the lower the fluid to air ratio is and the
smaller the droplets will be. The farther out it is, the higher the
fluid to air ratio is and the larger the droplets.
Set the air pressure at the inlet to the gun to the manufactures specs.
On an HVLP gun this spec is usually found on the gun and is the maximum
PSI it can have while still maintaining the maximum 10 lb at the cap for
legal HVLP transfer efficiency (68 %). You are now ready to do a test
spray out.
Tape a piece of masking paper on the wall for the test. Hold the gun at
a right angle to the wall, just as if you were going the wall. Hold the
gun at a spread out hands distance (about 8 or 22cm). Pull the trigger
to completely open for a split second and then close it. You want an
ON-OFF wide open-completely closed in ONE movement. You should have a
cigar shaped pattern with complete coverage in the center with fading
coverage going away from the full coverage cigar shape in the center.
The center should be fully covered without any runs. If you have runs,
either you are holding the trigger too long, you are too close or the
gun is simply applying too much material. In which case you need to
screw in the material knob or turn the air pressure down. But most
likely if you have turned the material knob out the 2 turns and the
air is set at the factory specs, you are just too close or holding the
trigger open too long.
The droplets you see trailing off the center are what you will use to
tune your gun.
Turn in the material knob to make the droplets smaller (and or raise the
air pressure). The balance you need to attain is the smallest droplet
size possible before you loose the coverage desired. In other words if
you turn in the material knob too far, not enough material will be
coming out to cover the panel!
Now, youll notice that I said, raise the pressure to the gun, while
earlier I said to set it to manufactures specs. We are talking a very
small adjustment. It is a fine balance in material to air ratio and a
little more air than specified is okay. Even if it is an HVLP gun the
inlet pressure recommended is to maintain the 10 lb limit at the cap.
Well, about three quarters of the country has no regulations for HVLP
use so if you go over the 10 lbs all it will do is atomize the material
a little better. You may loose a little of the benefits of HVLP though.
But remember you have a lot of control with the material adjustment
knob.
After you are happy with the droplet size, DONT TOUCH THE FAN CONTROL.
It will change the PSI at the cap and will change the atomization you
worked hard to get.
Do this spray out every time you spray as material change, temp, and
humidity will necessitate a spray out droplet pattern test. Good luck!
Check out this URL to see an example of a sprayout pattern.
http://members.aol.com/icantunderstand/hvlpdropletpatterns.jpg
Message 2
| INDEX | Back to Main INDEX |
| PREVIOUS | Skip to PREVIOUS Message |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
--> Rocket-List message posted by: "Jim Jensen" <sergeantjj@hotmail.com>
I have recently acquired an O-540 and had it rebuilt. It originally was
carb'ed and I want to go with injection. Can anyone point me in the right
direction as to a reliable make and cost?? thanks
Other Matronics Email List Services
These Email List Services are sponsored solely by Matronics and through the generous Contributions of its members.
-- Please support this service by making your Contribution today! --
|