Replies

1. 
   frenchy | Feb 12, 2007 12:22am | #1

   WGray,

    Spraying is a difficult task..   viscosity cups and other tools have so many cavots with them that I long ago gave up.  Temp, humidity, and a dozen other factors affect spraying, with practice it will come to you.. And each paint manufacturer seems to have a differant opinion about exactly what viscosity is ideal.. have you learned to alter your pattern to compensate for pressure fluctuations yet?   Just before the compressor kicks on you'll note a drop in pressure, not by a lot but enough that I bring the gun ever so slightly closer. I was taught to spray the pattern.  Move to adjust to pressure fluctuations then as the compressor kicks on, you've got to pull the gun back to it's orignal distance..

     I found a general rule is to over thin and then spray.    I like to get the pattern acceptable first and then worry about coverage..

     Using that approach makes things a lot simpler and I am not constantly looking for the chart.

    If you over thin then you spay another coat on.. far better to have a number of thin coats than one thick coat that needs to be repaired..  all you lose is a little time..  with experiance you won't even lose the time..

    You'll grab your gun, kick on the compressor, load it and pull the trigger..

    Only comes from eating a lot of paint fumes.  

    

         

2. 
   beachfarm | Feb 12, 2007 12:52am | #2

   First things first: What are you spraying?

1. 
   WGray | Feb 12, 2007 03:58am | #3

   I usually spray a primer of white lacquer undercoater then some variation of white enamel or latex (white, white and more white everone wants white.) I usually do all my other finishes by hand and occasionially spray General Finishes Pro Series Acrylic (that works pretty good).  I have results all over the place with the undercoater and the white.

1. 
   beachfarm | Feb 12, 2007 07:20pm | #8

   I can't add much to the chorus.For what it's worth: In have one gun dedicated to Duravar (cat lac). That's all it gets. I know how it handles, it likes a certain PSI, it never dissappoints. This makes it much easier to clean, too. Oil finishes go into another gun - same reasons. Ditto with shellac. I break them down for periodic cleaning and return them to exactly the same settings. Never any guess work. And it certainly helps that I'm loyal to a select few brands of finish.

1. 
   WGray | Feb 12, 2007 07:45pm | #9

   Thanks for everones input.  I realize I need to be more methodical with my approach.  It seems that I sleep too many times between spraying and tend to forget what I did the last time that it "worked" (old guy).  I'm going to take a little time and "practice" on some cardboard/scraps with different paint/mixtures before I tackle another piece. Thanks again.

3. 
   Kevin | Feb 12, 2007 04:20am | #4

   Ditto what Frenchy said for the most part. I worked at a place in the late 80s where the bossman decided that we needed to be spraying our solid color lacquer at a given viscosity, so he bought some viscometers and we started using them. What a royal pain in the backside! Pretty soon we stopped using them and he had forgotten by then that he'd laid down the rule in the first place - which was very typical of him. 

   Viscosity is important, don't get me wrong. But with experience you'll find that you don't need to know it much less measure it in order to achieve excellent paint jobs.

   I've learned that I not only like to spray thinner material and apply a supplimental coat, as Frenchy says, but I also like to manipulate the finish using different solvents, usually a retarder which helps it flow out nicely. For example, with nitro/CAB/precat lacquers and those few conversion varnishes which will allow it, I have grown very fond of thinning with MEK and PM Acetate rather than the specified thinner blend. The MEK flashes off quickly and keeps the finish from sagging or dripping and the PM Acetate, being a relatively mild retarder, keeps it open long enough to flow out nicely. I was actually introduced to this combo by the head painter at the place where we had to use viscosity cups. With other conversion varnishes I use a similar combo combining a compatible hot solvent with a compatible slow solvent to produce the same effect. I have yet to use a manufactorer's recommended thinner blend which works better.

   I read somewhere a while back about a guy who was using different alcohol blends to do much the same thing with Shellac. Heck, even the paint manufactorers have gotten into the act. Over the last several years virtually all of the big players in the latex paint market have reformunated and added some slow solvents to enhance flow out and other characteristics.

   Here's the thing. With whatever you are spraying, if it's the same stuff every time then just use an empty gallon can and eyeball the reduction schedule. If filling it 3/4 of the way full with your paint and then the remaining 1/4 with reducer gets you a finish which applies and behaves the way you want it, what difference does it make what the viscosity is? Or... let's say that you do figure out your viscosity cup and arrive at a reduction schedule which yields the specified viscosity. You're not going to need to measure it every time as long as you remember what reduction schedule got you the results. You'll know how much reducer in a given amount of material achieved the specified viscosity and thereafter the viscometer will sit in the corner collecting dust because you don't need it. So just cut to the chase.

   The automotive paints and clears I sometimes use don't even specifiy viscosity. They are all predicated on a ratio of material to catalyst to reducer. It doesn't matter whether I'm mixing a pint or a gallon. All I have to do is get the ratio correct. There's no reason in the world why wood finishes or any other finish/paint can't be done the exact same way.

4. 
   hammer1 | Feb 12, 2007 05:16am | #5

   The Finishline gun I have requires a lot of disassembly to clean properly. Mine is a syphon style cup. You need to pull the fluid tip baffle and seal as well as the packing nut at the trigger along with the seal and bushing behind. I've also found that the connection between the cup and gun can get clogged and corroded.

   I spray a lot of different materials from acrylic and oil paints to automotive candies and clears. I make most of my adjustments with air pressure rather than worry about viscosity. I try not to thin materials because it can change the sheen and may effect how things lay down. I might be running 60PSI at the compressor with thick materials. I might also have the air and fluid pretty close to max at the gun. You have to be cautious of too much blow back with the air pressure increased. Gun distance and speed are always the demanding part of spraying. 8" is about right for distance. Pull the trigger and start the movement off the work and move into it. Don't pull or let off while pointed at the work, you may get spitting crud.

   I would double check that everything is clean and undamaged and try a little variation on the air supply pressure. You may have a seal repair kit that came with the gun. I normally adjust my fan pattern on cardboard by starting open and closing down. It doesn't take much movement of the adjustment knobs to fine tune but it could take a 3/4 turn to get started. It does take plenty of practice and concentration.

   Beat it to fit / Paint it to match

5. 
   polarsea1 | Feb 12, 2007 08:50am | #6

   It's true there are a lot of variables but it's nice to have a baseline or benchmark to start from. You can get a Ford #4 viscosity cup from Homestead Finishing where you can also find a viscosity cup conversion chart posted in the techinical section. And that's probably your best source of information about finishing: Jeff Jewitt and the forum at Homestead Finishing.

   Call the tech line for Devilbiss; give them the viscosity of the material you are shooting and they will recommend a tip and aircap.

   Good luck.

    

6. 
   InMyOpinion | Feb 12, 2007 09:00am | #7

   To go directly to your question, apollosprayers.com will sell you a relatively inexpensive viscosity cup. To be certain, *ask* for the specs; they are within its sprayer manuals. Consider that it is not a Zahn nor a Ford measuring device per se. The spec sheet will state, for example, that if the flow stops at 32 seconds, it is an "x" Zahn value, and will allow you to read across and convert 32 seconds to a "y" Ford figure.

   It is tricky providing a specific answer about a "wrong finish." I don't know what that means. My comments pertain to my HVLP experience with poly, latex, and waterborne lacquer. I will keep comments simple and general, as I think the approaches to and discussions about finishing and painting are unnecessarily complex. (The nomenclature, chemistry, physics, and other variables such as viscosity, temperature, spray guns, etc. are complicated. But doing the job does not have to be.)

   Quality HVLP results using various products, i.e., liquids and hardware, require a methodical, documented approach. Comparable results can be obtained more easily with airless or quality brush applications (and a conditioner in the case of latex paint). Start with one product in the cup of one gun set at a constant fan to be shot at a constant distance. Use a compressor (or turbine I suppose) that can keep up with the gun—so that the pressure to the gun is always lower than the tank pressure.

   You decide the remaining variables if you like. I believe they are viscosity (including product and ambient temperature); tip and needle size, pressure, and flow rate. Now, the trial and error begins . . . but it is controlled. Early on, you need to learn/know how you want the liquid to look after it hits the sprayed surface (such as cardboard or painters' paper) after one or two passes. Obviously, you are aiming for something between a cloud and splatter. Once achieving something reasonable, spray a few coats, sanding in between. (Each coat may consist of several passes). Once obtaining the finished product you like, you have taught yourself how that product should behave when sprayed. This is a critical part of the entire drill because, generally, what you conclude will be applied to every type of liquid you spray.

   Before I explain the fine-tuning, please understand that if the liquid is of low viscosity, such as poly diluted 50/50, a relative small tip and needle with reasonable pressure should get you very close to quality right away. Remember, though, this answer began with a question about viscosity, and I am sure the problems are with higher viscosities.

   A viscosity cup is unnecessary. If the liquid takes more than 10 seconds to stop dripping from a paint stick, you are in trouble. Four seconds should be easy to manage; but, that may mean severely diluting the liquid—not good. Temperature affects viscosity somewhat. If you are operating in extreme temperatures, consider changing the gun's cup temperature with a water bath.

   Setup a matrix and begin recording what you are doing during your fine-tuning. Column headings might include:
   > Product name
   > Approximate temperature
   > Ratio of product:diluent:conditioner
   > Fan (full, 2/3s, whatever)
   > Needle (which should have a constant, corresponding tip and cap for your gun) size. Sadly, the size of the spray cap, tip, and needle are not consistent among gun manufacturers—compelling us to think, here, in terms of small, medium, and large sizes.
   > Flow rate (typically a number of screw turns)
   > Pressure

   Soon, I think you will find that pressure and flow rate will be your key variables. Viscous fluids tend to require more pressure for atomization. You are constrained by concerns about overspray and about compressor output limitations. As much as you would like to spray at a reasonably high flow rate (to get coverage and maybe even some leveling), higher flow rates often impede atomization . . . orange peel being a common consequence.

   So, you keep working on it, keeping track and grading the outcome. As you go, you can make tweaks (a narrower fan atomizes better, for example). Lastly, consider trying to talk with someone close to the producer's varnish/paint lab, i.e., a scientist vs. a salesperson. If you are aiming for perfection with your spraying, it is reasonable to assume you may be asking for more than the varnish/paint can deliver when sprayed.

   
   

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   Edited 2/12/2007 1:03 am ET by [email protected]