Gravity or Suction Spray Paint Systems?

[Lionelgee]

Hello All,

I am thinking of making the step up from using pre-mix aerosol spray paint tins to the real McKoy. From a quick look online there seem to be two main types: gravity fed, which has the paint pot above the spray gun nozzle; and suction/siphon, that has the paint pot below the spray gun nozzle.

I already have a compressor I have a range of pneumatic mechanic's tools - just not a spray paint gun yet.

I would be mostly painting things like chassis and I might have a go at panel painting. I am a novice and do not intend doing spray painting for a living.

I also noticed that some spray systems vary in price from $34 for a kit; to well over $500, just for the hand piece alone.

What are the main differences and applications between gravity and suction - is one more advantageous than the other?

Is one type more flexible than the other and better suited to smaller - occasional jobs?

Is High Volume Low Pressure really the duck's guts like some of the advertising material suggests?

What are the brands that people would recommend for each type gravity/suction?

What sort of cost would get decent quality system?

You know a spray gun where I am not encouraged to practice my swearing due to shoddy results directly attributed to tools with poor adjustments or weak componentry, awkward to hold. I have to compensate for my own lack of ability after all.

Your thoughts would be greatly appreciated.

Kind Regards
Lionel

[Private_collector]

Wow, that's a lot of Qs.

I only use gravity feed. There are many benefits of gravity over suction. The first one is simply the ability to spray in a very wide range of angles, and because no pot hanging below the gun, you won't accidentally drag it through the fresh paint while applying anothet coat. Imagine painting a roof panel, reaching across to mid line, and trying to fight against fatigue. Don't laugh. After the first two coats, that ackward position becomes rather tiring. Gravity feed allows you to get every last bit of paint out of the pot. You don't have the fear of pot coming loose or leaking when gun is tilted. The air hole on suction feed is always ABOVE the gun, so it is less likely to drip out, if the cap is turned towards the user. People assume the air hole on suction won't leak because of the negative pressure (suction), but this is not always the case in practice.

The High Volume Low Pressure (HVLP, from now on) gives a much more consistent paint flow, and therefore, a noticably better result. HVLP is used almost exclusively when refinish shops are using the Poly paints, 2-Pack, etc.. I started my painting years with suction. The HVLP guns weren't common at that time, because we mostly used acrylic or plain old enamel (or 'jam' as we called it then). Once repair and resto shops started using the far superior polys, we went to gravity guns instantly. The range of fan adjustment and airflow will suit any situation. Different locations require minor changes to gun settings. I'm contantly adjusting my gun during use. If you are using acrylic or enamel, suction will do, but you will most definitely get better results with gravity (HVLP). As the name suggests, you don't waste any of your air supply in trying to draw up paint from botom of a pot! That suction level changes slightly as your full paint pot starts to empty. You won't notice it, but the gun will behave differently, I guarantee it.

There are many brands of quality spray guns and equipment. Devilbiss, Iwatta, CIG, are just a few. You can pay into four figures for some of the very best guns. Most of the better brands will set you back in the vicinity of $200-400. Having spruked the virtues of gravity guns, and how the finish is better, in my opinion, I'm about to say something that sounds TOTALLY contradictory to chasing the best result. All of the spray work on my restoration has been done with a gun that cost less than $50, from Supacheap Auto. Now, I'm working on the assumption that you don't have a sterile spraybooth, and are not planning a full respray on an Aston Martin, so a gun which is setup correctly, and sound technique is all you will need to produce a great finish. Seriously! Everything I have painted so far, has been done outside in the elements, and I don't think anyone could tell. Some of the credit goes to a lustreless paint. Very little shine equals much reduced reflection, therefore any nibs in the paint don't draw the eye like gloss would.

I know you're a fair distance from me, but if you are around this way in the near future (at least before you need to start painting), get in touch and call in. I'll show you how easy it really is to setup a gun correctly and get a good finished product. Once you are aware of WHY certain adjustments and technique are done, the rest just falls into place.

So, in short, don't waste big bucks on a spraygun, but I would strongly recommend a gravity feed gun.

[Lionelgee]

Originally Posted by Private_collector

So, in short, don't waste big bucks on a spraygun, but I would strongly recommend a gravity feed gun.

G'day Tony,

Thank you for your detailed reply Tony. It is much appreciated.

Lots of questions ... I will quote Snoopy, "It is better to ask dumb questions than make stupid mistakes".

Also I am working from a point where I know only what I have recently researched online. When I was young the only option was the suction type spray gun.

Kind Regards
Lionel

[Phil Waterman]

Hi Lionel and Tony

Would you guys mind if I use both your question and response in local club newsletter, because they really summarize the issue and advice around painting military vehicles.

Cheers Phil

The only thing I would add is protect your lungs, but that is a whole separate thread on shop safety.

[Private_collector]

Fine with me, Phil.

I didn't go into WH&S rant, because Lionel knows that stuff already, I expect.

[Lynn Eades]

Tony, Can you give us a run down on setting up a gun.
You are able to offset your cheap gun with your experience. A lot of us have the cheap gun and no one to guide us. A general run down on those two knobs on the back of a suction gun would be helpful. What is different on the gravity gun? What do we look for?
Thanks.
Lynn.

[Lionelgee]

Originally Posted by Private_collector

Fine with me, Phil.

I didn't go into WH&S rant, because Lionel knows that stuff already, I expect.

Hello Tony and Phil,

It is fine with me to copy the post.

As a former horticulturist I have sprayed lots of chemicals and I am aware of its related OH&S. So Tony is correct about that

Kind Regards
Lionel

[Private_collector]

Originally Posted by Lynn Eades

Tony, Can you give us a run down on setting up a gun.

Sure, Lyn. There's no trick, it's just a matter of finding the right setting for each combination of gun and paint type. Acrylic paint will require entirely different settings to enamel, different again to 2-pack, etc. If, like me, you paint outside, temperature, wind, direct sunlight or shade, and paint type all combine in dictating the end results. Might be easier to explain if I use illustrations and possibly demonstrate what the various adjustments do. On Sunday I will find a suitable bit of metal to paint on, and I'm sure I have some usable paint that I won't use for anything, which I can put into the gun for the purpose. Bear with me until then. I recommend you find a nice big panel to practice spraying on, and experiment with changing gun settings and air flow rate, using the type of paint you intend to use for the 'real' job.

There are some things I will take as a given. Always follow manufacturers instructions, with regards to thinning ratio, and hardener additive, if applicable. Always clean the spray gun and pot immediately after you finish spraying. A dirty or partially blocked gun or nozzle will never give a good result. Usually any blockage can be cleared, but this may involve immersion of the gun for a period of time. I do the first clean of the gun with a small quantity of the thinner or reducer which is applicable to the paint which had gone through the gun, then another, more thorough, clean with thinners which I know as either "G.P thinner (general purpose) or "Gun Wash". As the name suggests, the latter is only good for cleanup of equipment, not for thinning paint. I recommend buying a multi scale measuring stick AND graduated (marked, not received a qualification!) mixing cup. Most cups come in a range of sizes, to suit any quantity of paint desired, and should be clear so you can watch the level of material from the outside. They usually have several different ratios marked on the sides. Mine I think have six ratios 1:1, 1:2, 1:3,........ you get the idea. The measurements usually include the ratio or hardener, if it is required. I cannot overstate the importance of mixing ratios. I know it is a pretty basic consideration, but you would be surprised how often i hear someone say they mix by 'sight' or close enough. Every part must be exact, at least until you know what you can achieve by altering the ratios. I won't go into that for now. Best left for later.

I'll have a little play on Sunday, and post some info with photos. As i said before, there is no great mysteries, just some principles which must be observed, no matter what type of paint you are using. Those principles, once fully considered, will get a good result every single time.

I recommend everyone wanting to refine their skills prior to the real paint job, to get their hands on a nice large vehicle panel. A decent size bonnet / hood would be perfect to practice on. Stand it on it's side, and spray your practice when the panel is vertical. There is no challenge in spraying a panel when it is laying horizontal. Paint won't run off it like it will when it is standing vertically. You will want to ensure you can avoid paint runs, so make your practice reflect the conditions you will be subjected to with the real thing. Once you can successfully spray without dry spots or runs in the paint, repeat the test again with straight thinners, no paint. If you can apply thinners evenly and it doesn't run, then you have achieved your goal. Any spray job after that should be a walk in the park, and you will be ready for the task at hand. It will make more sense when I demonstrate, trust me.

Excuse any spelling mistakes in the above. I don't have my glasses on!

[Private_collector]

As promised..

This is my gravity feed gun. I have used this exclusively for past three years. Provided I keep it well cleaned, it should see me through to the end of the restoration\s. It never misses a beat. One curious eventuality though, the 'chrome' plating is coming away in serveral areas where my hand holds the gun. I believe the sweat penetrates to the metal beneath, and causes breaking down of the surface finish. Doesn't bother me. Nothing falls off onto the fresh paint.

The paint volume adjustment is the knob at the back of the gun. In the exploded photo, you see the fluid needle and spring assembly. It works in the same way as a needle does in a carburetor. Turn the knob fully in, paint flow is ceased. The volume of paint being atomised increases as the knob is turned out. When using the Protec Barrier Enamel (I also use hardener) I turn the knob three full rotations. I like a LOT of paint coming out. You get less dry spray that way, provided the correct ratio of reducer is used. The adjustment would be different for other type of paints, and other individuals.

Now, because I want a high flow of paint, I need to increase the spray fan pattern. That is the round knob in the upper middle of the gun, on the side facing camera. My spray path height is around 13cm, or 5" for those in non metric countries. If I need to paint something restricted, say, inside a tube or somewhere hard to reach, the fan pattern should be narrowed significantly. Air flow should be reduced heaps too, or you will send such a blast of paint in there that it is bound to run everywhere. Depending on where that place is, you might WANT a heap of paint in there, especially if you need good surface coverage without being able to see there after everything is put back together.

Speaking of air flow rate, my gun has air flow adjustment knob at the base of the hand grip. Some don't. If yours doesn't, get an attachment to put between the gun handle and the air hose fitting. Most of the new ones have a swivel, so you don't get the air hose hanging down further and dangling in your fresh paint. That's no fun at all. In fact, that's another reason why I like gravity feed guns. You don't have a big pot hanging below the gun, just waiting to either swipe a nice mark through the paint, or leaking paint out of the breather hole. If you are really lucky, the pot may even come off entirely. That is hilarious, provided you are an observer, and not the one doing the painting. I have seen this occur from both sides of the fence. With a gravity gun pot, it's always above the gun (naturally) and cannot drag through wet paint. In the above exploded view, you can see the little blue cap that pushes into the pot lid. It can be turned 360 degrees. Unless you are spraying something from underneath, the breather hole in that cap should ALWAYS face the back of the gun. Don't forget to reverse that if you DO want to paint upwards. You can alter the breather hole on a suction gun pot too, but because of the sloshing movement of paint inside the pot, drips are a common occurrence. Once again, this is only funny when it's your paint job!

Having mentioned the settings I prefer, I should advise the importance of keeping the spray gun at a 90 degree angle to the surface being painted. This is VERY important, and cannot be stressed too greatly. If your panel curves in any way, you must match that angle change with your spraying angle accordingly. The only time I ignore this rule is to either blend color into another panel (referred to as Wetting Out), if I am applying over thinned paint, or deliberately trying to reduce gloss level. This can be achieved by spraying a light coat with considerable additional thinners / reducer. On that subject, ALWAYS use the correct thinner, reducer, hardener, and flattening base for the paint you are using. Be sure to ask the paint shop for the Tech. or Spec. sheets for your chosen paint. It will tell you mixing ratios, surface preparation, interval between coats, total drying time, cleanup, and most importantly, SAFETY REQUIREMENTS. if you spray some paints in an enclosed area, without the correct PPE, you can die! You won't know how good your spray talents were, if you are too busy being dead at the time. Some paint gives off isocyanates, which is akin to cyanide gas. Lungs eaten away by inhaling that stuff will be permanently damaged. Ask someone with emphysema how they feel.

These are the mixing cups I like. Very easy to see the ratios, and there are any ratio you will come across, marked on the sides. They're easy to clean too, and double as something to put all the parts of the dismantled spray gun into, once painting has finished. I only use a metal stirring stick. They won't shed little bits of wood, won't split, clean exceptionally well (even if you let paint dry on it, just scrape it off later), and also have mixing ratios, if you don't want to buy the plastic cups. Both the stirring sticks and cups are very inexpensive. So are the disposable cone shaped strainers. I don't worry about contaminants in the paint until I'm ready to pour into the paint pot, then I sit a strainer into the pot and pour away, with no fear of muck ruining a good paint job, OR blocking up the gun from inside. I also like the screw on pouring spouts. Much better control of volume coming out, with those, and also use a clip on pouring lip for the paint cans. Dont forget, NEVER return mixed paint to the original can. If you have extra, put it in another clean can. If you used hardener, you may be able to use it for a number of days, depending on temperature and amount of hardener added, it will congeal in near future. Better of using any extra paint by putting another coat on your project, or use it to brush onto bolt heads, screws, surfaces you can't get spray onto, etc.....

One final piece of magic. Tac cloths are a sticky gauze material, designed to be lightly wiped across the surfaces to be painted, after they have been blown clean of dust etc. and then cleansed with Prep Wash or similar wax & grease remover. When I say 'lightly' wiped, I mean it. Rub too hard and it will put the sticky stuff onto the surface. You'll know when that happens. Flys will get stuck! The Tac cloth comes in white and yellow, which are both colors which show how much grit and dust has been collected from the supposedly clean surface. You would be amazed how much you will see. Do be sure not to buy a Tac cloth in the same color as your primer (or whatever you are applying paint over), or you won't see how successful you were with the initial cleaning.

Spraypainting is easy, so long as the technique is sound and the basics are understood. Having a lot of practice helps too. There is a huge number of hints and tips which can make the job sooooo much easier. You could fill books with em, which I recommend you buy if you want to excel. I have seen really professional results achieved by amateurs, and dreadful work done by professionals. Next time you are travelling in the roads, keep watch for cars that have panels which dont seem to be quite same color as the rest of the vehicle. These shoddy jobs are everywhere, and it amazes me how low the customers standards must be to accept the repair work.

Enjoy.

Tony