Gun Cleaning

I think cleanup is the worst part of painting. Maybe there's a better way than I'm using. . .

Also, if I could squeeze another question in: when I'm applying successive coats, should I just follow the spec sheet (5-10 min dry time) or do you test for tackiness, or both?

Thanks

I don't claim to be any kind of expert but I will offer an opinion. While I have been painting on and off for quite a few years, I am still learning new stuff all the time. Matter of fact I will be doing my very first BC/CC effort here in the next couple of weeks on my wife's replacement car bumper.

Your HVLP gun (assuming gravity feed) should be the easiest to clean of any gun there is. That's one thing I have learned fairly recently. While I don't particularly like the gravity feed aspect of these guns, I admit that they are very easy to clean.

You do not have to take any cleaning action at all between coats of the same material. The 182 with the short pot life could possibly be an exception. If you have to use more coats than you can apply within the pot life time, you should at least stop and mix up a new batch of the surfacer. The safest way would also be to clean the gun at this point.

The most important gun cleaning tasks are those that get done between a change in painting materials (e.g. going from primer to topcoat) and the final cleaning at the end of a daily session.

In the past I have always just run gun cleaner through my guns several times between changing materials and disassembly only at the end of a painting session. The HVLP world is fairly new to me and I have been using an HVLP spot gun for a couple of years now. I have not only been running cleaner through it several times but also disassembling it between a change of materials. I have done this because I almost ruined it trying to spray Loctite's Extend Rust Treatment once without complete disassembly. I have just begun using a "percolating" procedure and bypassing the disassembly between change in materials. See my recent thread below titled "Percolating". This procedure seems to work well although I haven't evaluated it in all situations. I will not use Extend with the gun again. It is not intended for that material anyway. I think you should still disassemble the gun at the end of a painting session (or day) and do a thorough hand cleaning. By this I mean removing the fluid needle, air cap, fluid tip and any seals around the fluid tip, soaking everything in gun cleaner and making sure all passages are clean, including the orifices in the air cap. If you did not get some cleaning brushes with your gun, I would recommend you purchase a gun cleaning kit that has these brushes, of various sizes, in it. They are used (with gun cleaner) for cleaning out passages in the gun body. The HVLP gun I have is pretty simple and easy to do this to. It only takes a little time and is well worth the extra effort, IMO.

I believe the flash times given in the tech sheets are a good guide but can be used with a little leeway. They are usually given at 70 deg F temperature. I consider them to be a minimum at that temperature. If it's cooler than 70, give it a little more time between coats. I never cut the times short, even at much warmer temperatures. If the flash time is said to be 10-15 minutes, then letting it set for 20 or 25 will not be harmful even at warmer temps. But be reasonable in this. I would certainly not wait an hour between coats. I don't do any kind of "testing" between coats although I admit to sometimes touching lightly in an obscure area, more out of curiosity than anything else. I have always found that the flash times stated in the tech sheets work well. It is more important that you use the correct reducers and hardeners for the temperature at the time of spraying and follow the stated flash times than trying to evaluate how tacky it is.

I know gun cleaning is a PITA. Just get used to doing it religiously. You will only have to deal with one gun that has hardened urethanes in it and you will consider routine cleaning to be an integral part of the painting process. Consider it right up there with scraping hardened grease off with a putty knife, or a lot of hand sanding...nowhere near as much fun as watching the topcoat being sprayed on, but just as necessary. I'd much rather clean a gun several times than deal with rust pits...if that makes you feel better :o).

Rod

Thanks for your reply - your posts have all been very helpful to me. I'm painting (MTK) everything but the hood and fenders of my 51 Farmall Cub today so I wanted to make sure I asked all the questions first. I'm saving the hood and fenders till last and my technique has been refined a bit.

Just a couple more questions if you don't mind:

1. Do you know if the percolating procedure you spoke of would work for a pressure feed HVLP gun (what I have)?

2. You mentioned you have a HVLP spot gun. I assume that is one of the small (maybe a pint cup?) guns. Could I use one of those to touch up small spots after the main topcoat has been applied - for example bolt heads after individually painted parts have been assembled? Will it blend in?

3. I've heard people say that they never spray primer and topcoat with the same gun. I noticed that after half a dozen sessions with 182, the needle tip (1.3mm) of my gun shows some erosion. Is that to be expected? (I know there is silica in the 182 - could that be the reason?) Don't know if it would make a difference, but I purchased a new needle for the topcoat just in case. Also, based upon other recent posts I bought a larger tip set for the 182 in the future (1.8mm).

I appreciate all of your help.

Paul

As far as I know, the percolating procedure is only appropriate for gravity fed guns.

My spot gun has a 4 oz cup. It's great for all of the smaller parts encountered in tractor work (say less than about 2 or 3 sq ft each), assuming you take things apart and do individual pieces. The one I have has a 5" or 6 " fan pattern although most others are more like 3" or 4". It would be too large for the touch-up type work you mention. For that you would want an airbrush or just a manual artist's brush. I have used a 1/8" artist's brush for touch-up of bolt heads and nuts after assembly. It's not absolutely perfect but it works quite well.

The ones that have separate primer guns are usually those that paint on a daily, or at least frequent basis. I have never had a dedicated primer gun...but then again, I don't paint often. I have never noticed a problem with tip erosion although I have read about it. Course most of my past surfacer work has not been with the 182 but with lacquer type surfacers. Not sure if there would be a difference there regarding erosion. My full size gun is an old DeVilbiss JGA with a hardened alloy steel fluid tip and a stainless steel needle. The tip size is 1.1 mm. I use that gun in a pressure-fed, remote cup arrangement for all materials, including surfacers. It is a conventional gun, not HVLP. The fact that I use pressure feed means that I don't have to change tip sizes for different material viscosities...just adjust the pot pressure. The 1.1 mm size would be too small for a surfacer if using gravity or suction feed. I have tried running the 182 through my HVLP gravity spot gun (1.0 mm tip) and it worked but not well. The tip was way too small for the 182. It works fine with epoxy primer though.

If you are accustomed to using pressure feed, try out a remote cup arrangement sometime with your gun (assuming you don't already have it). I think it's the only way to fly for larger jobs. Most are 2 qt capacity and you can cover an entire full sized car w/o stopping to refill. Plus you don't have the paint weight in your gun hand and you have excellent all-position painting capability. Great for stretching over wide car hoods and any upside down or awkward position work. Tip sizes with pressure feed are determined more by the desired material flow rate than the viscosity, since the material is being supplied to the tip under a positive pressure of between 5 and 15 psi, depending on pot pressure setting. The pressure-fed 1.1 mm tip that I use will flow any automotive paint as fast as I want to go. It will flow enough material to yield a 16" fan pattern if you have the appropriate air cap and compressor capacity to properly atomize it. For the same gun in suction feed from an attached cup, the standard tip was a 1.8 mm which is large by today's gravity HVLP standards, as CNKS indicates. They didn't have gravity feed in those days. I would expect that a gravity gun would need a smaller tip than a suction gun, all else being the same. Make sure you check the documentation that came with your gun. Material, tip size, air cap and operating mode (pressure/suction/gravity) all work together. A 1.4 mm tip would be considered a huge tip for pressure feed, too small a tip for suction feed and about right for gravity feed, considering a full sized gun using an 8"-10" fan pattern. At least that is how I am looking at it.

Rod

With a pressure-fed setup there are two different pressures. One is the normal air pressure that provides atomizing air to the air cap. The other pressure is also air pressure but it has nothing to do with atomizing the paint and does not go to the air cap. It serves only to pressurize the paint in the cup, either attached below the gun similar to suction feed or remote from the gun with a remote cup arrangement.

With either a suction or gravity feed gun, HVLP or not, the paint is under zero pressure - the cups are vented to the atmosphere. With pressure feed the cup (and paint in it) is under pressure, whether the cup is attached or remote. This pressure is provided by a separate regulator from the one suppling atomizing air. The paint is therefore forced to and through the fluid tip under pressure. It does not have to just flow by gravity or be sucked up a suction tube.

With pressure feed, the atomizing air pressure for non-HVLP is, as you say, 40-60 psig at the gun inlet. The fluid (paint) pressure is the pressure in the cup and is separately adjustable. Typically this fluid pressure will be between 5 and 15 psig, depending on fluid tip size and paint viscosity.

Here's my pressure-fed remote cup arrangement:

The air from my wall regulator goes in the quick coupler nipple shown. From there it goes to two things. One is the atomizing air to the gun via the red hose. At the gun end of the red (air) hose is my inlet pressure gage, PG 1. I set my wall regulator to whatever will give me the 40-60 psig on this gage. I usually like to go for the high end, nearer 60 since I like a fairly high rate of paint flow and need a little more atomization air. The second path for the air is to the regulator for the fluid pressure, REG 1 shown. This regulator is set to establish the paint pressure in the cup, shown at PG 2. I usually set it about 10 psig, closer to 15 psig for a surfacer. The paint is forced under pressure through the black fluid hose to the fluid tip in the gun. There is very little air flow through through REG 1 - only that amount that is required to maintain the cup pressure as the paint level drops during use.

I have absolutely no idea what the actual air cap pressure is for the atomizing air. I am sure it is significantly higher, perhaps a lot higher than the 10 psig mandated by HVLP. The 10 psig HVLP requirement is "at the air cap". It is outside the fluid tip and not inside it. The actual pressure inside the fluid tip is slightly sub-atmospheric for a gravity feed gun and more so negative for a suction feed gun. A pressure feed gun on the other hand has a positive pressure inside the fluid tip. A lot more paint can be put through a small tip size under pressure than can be put through the same tip by gravity or suction. Think of it this way: an HVLP gravity gun usually has one air cap that is designed for a certain fan pattern with a single inlet air pressure and a constant paint flow in oz/min. Materials of different viscosity are accommodated by changing fluid tip and needle sizes in order to get a similar oz/min of paint flow. A pressure feed can use different air caps for different fan patterns, different paint pressures, different air atomizing pressures and can accomodate most automotive paint viscosities with a single, relatively small fluid tip and needle assembly.

Yes, the PPG P-sheets (both OMNI and premium lines) do not indicate any pressure feed information. I suspect it's because that type of arrangement is not popular in body shops although I would think fleet painters would love it. There are also less variables to deal with on a gravity feed HVLP setup so compliance with the law would be easier. Interestingly, the DuPont P-sheets for the various mixes of Centari acrylic enamel indicate that a pressure pot (pressure feed) is actually the recommended mode but do not give any setting information. The only pressure info they give is for a siphon gun (suction feed). I suppose that is understandable since putting "8-10 psi at the cap" for a HVLP gun really serves no useful purpose in a paint spec sheet and could add to confusion...the user has no realistic way to measure it and has to depend on the gun manufacturer to provide the correct gun inlet pressure for legal compliance. With two air pressure regulators, one for atomizing air and the other for pressurizing the paint, there can be somewhat different settings between them to accomplish the same thing. If I increase the paint pressure in the cup I will be flowing more oz/min of paint through the fluid tip. To properly atomize the greater amount of paint, I will also need to increase my atomizing air pressure. Rather than attempting to provide specific settings in a paint spec, I would guess these settings would be best determined in the field for each individual's situation. I usually start with a fixed atomizing air pressure and increase the paint pressure until there is a nice full pattern with good atomization. The setting will also change if a different air cap is selected in order to provide a larger fan pattern (more paint flow required).

I hope this explains it. It's a real blast to paint with this arrangement although it is not appropriate for small items that can be done with a spot gun. The biggest down side is that it is a PITA to clean. Not only do you have the gun and cup as separate and distinct pieces but you also have to flush out the black fluid (paint) hose. Plus it is more complicated to adjust the two different pressures correctly.

I don't know exactly what gun arrangement Paul has. Apparently it is a pressure fed gun or at least is capable of being pressure fed. Pressure fed guns with attached cups are usually also suction feed guns. Mine originally came with a suction cup but I replaced it with a remote one. In other words, you can use the gun either as a suction feed gun or as a pressure feed gun but you would usually have to change fluid tips and needles to accomodate the mode selected. At least I had to. I have never seen a combination pressure feed/gravity feed gun but they may exist. It is certainly possible to do given the right pieces. If he is actually pressure-feeding the gun, the 1.3 tip may be too big for the 182 depending on his air cap and fan pattern. In other words he may be flowing more paint than his air cap is designed to atomize properly. There should be charts that came with his gun for some guidance in setting it up in different modes for different viscosity materials.

Didn't mean to write a book here but I get carried away easily :o).