Teach Me about Spray Guns

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jjh
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Teach Me about Spray Guns

Post by jjh » Mon Sep 07, 2009 9:07 pm

I've recently read Dan Erlewines Finishing book I have a small 7.5 cfm compressor with a 25l tank, and a standard cheap gun, I'd like to spray sunbursts... Now in the book he suggest a jamb/touch up gun I was loaned one and really couldn't make it perform satisfactorily, then I looked at the spray guns available from Stew Mac and compared them to the guns on ebay, and there is a huge difference in nozzle sizes.

The Stewmac Touch up gun has a 1.6mm medium flow nazzle where as most guns of a similar size have between a 0.8mm-1mm nozzle I've only found 1 gun that has a 1.4mm nozzle I think this is why I struggled with the gun I was loaned, now Stew Macs full size gun had a 1.8mm medium flow nozzle where as most full size guns I can find including mine have a 1.6mm nozzle, the only reason I hesitate using it is that if I was spraying a sunburst with it I'd need to mix so much stain to stop the gun spluttering as it's suction and needs to be a 1/3 full to operate ok.

This there are a few guns on ebay I was looking at, out of the ones below which do people prefer?

http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... K:MEWAX:IT
http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... K:MEWAX:IT
http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... K:MEWAX:IT
http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... K:MEWAX:IT



Anyone else come across this?
All hints tips advice welcome

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Nick
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Post by Nick » Tue Sep 08, 2009 5:59 am

First off apart from the first gun in your list of links, the rest are HVLP (High Volume Low Pressure) guns which require a different "compressor" than the one you have. I say "compressor", they aren't really a compressor, they are a turbine that moves a large quanitity air at low pressures (hence HVLP guns). So this limits your choice to the first gun if that's the way you want to go.
Do you intend doing alot of bursting (on guitars to follow) or just a 'one off'? If it's a one off then that gun would do the job but you get what you pay for and I suspect long term the ability of this gun to produce consistent quality results would be low. If you intend doing a few bursts then spend a few more reddies and get a good quality gun, it will pay for itself when it is still giving good results after many bursts.
You say you weren't getting along with the gun you were loaned to try with.More practice will cure this, fill the cup with water and practice, turn every available knob (one at a time) and see what effect it has to the spray pattern you are getting, it's the only way to familiarise yourself with the gun. Once your familiar with it by using water then dry the gun and refill with the medium you intend to use, get a piece of panel board/MDF or something 'expendable' with a large surface area and adjust the gun to get the pattern you want, now you know what effect turning the different knobs has to the fan then it will only be a matter of adjusting the amount of air going through the gun and spray fan width.
You also mentioned you got alot of sputtering, the main causes of this are:
A) Not enough air pressure on your compressor's regulator (between 40-60 PSI for most mediums).
B) Medium in the gun is too thick, thin it down!
& C) A dirty gun. Take the gun apart and clean it, soak everything in solvent to loosen up any old crusty bits and carefully wipe everything until it's spotless.

You say in your post you had to have the gun at least 1/3 full before it worked properly. With a door jamb gun (which is the one you want for bursting),the cup should be at least 3/4 full when you start spraying. Especially with stain if you are spraying directly onto the bare wood. It's no use taking the economical approach and thinking "that is all I want" because you will never know how much you want until you see it on the instrument and the right shade. If you are using just stain through the gun then any excess you have in it after getting the correct shade on your guitar can be poured back into the container your stain is in. If you are using stain mixed in with a clear finish (laquer e.t.c) then unfortunately any left overs will be wasted and disposed of, it's just the nature of the beast. Door jamb guns don't hold much in their cups so any excess will not be that much anyway.
Hope this sheds some light on you question, Allen is the true spraying expert, it's his profession so he may chip in here.
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jjh
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Post by jjh » Tue Sep 08, 2009 7:58 am

I no my compressor isn't all that powerful bit it'd run both the first three guns ok or it has enough cfm for those ones, and I didn't know with the last gun if I could just wait for the compressor tank to fill instead of continual spraying.

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Bob Connor
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Post by Bob Connor » Tue Sep 08, 2009 8:02 am

Bob, Geelong
_______________________________________

Mainwaring and Connor Guitars

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Nick
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Post by Nick » Tue Sep 08, 2009 8:12 am

I knew the guru would have a post somewhere 8)
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Post by Allen » Tue Sep 08, 2009 8:38 pm

It's almost impossible to tell anything about the guns you've posted from the advertising blurb on them. My personal choice will always be with a gravity feed gun.

Reason is that even without air to the gun, when you pull the trigger, fluid will come out the nozzle. You might think big deal? But the difference between suction and gravity feed is a much more efficient use of the available air to the gun. All the air going to a gravity feed gun is used to atomize the product, whereas in a suction feed gun about 25% of the air pressure is used to pull fluid up the pickup tube and get it to the air cap, where the remaining air is used to atomize the paint. IE, more efficient for given available air flow and pressure.

Another plus is that every drop of sprayable material is available. Not so on a suction feed gun.

While it use to be true that you would need a turbine to supply air to an HVLP gun, new designs have come about where standard air compressors will drive these guns. With that in mind though, I'd take the HVLP rating with a huge grain of salt, especially in guns that cost less than $500. It's just a whole lot of advertising bull shit, and at the end of the day, isn't going to make a lick of difference in what you are wanting to do.

The sizing of nozzles is also a real grey area. When you get into high end automotive spray guns from a know manufacturer, then you can get an idea of what a size change will mean. Even between high end brands there will be a correlation. With consumer graded guns it's just a shot in the dark though.

For example, my everyday "Go To Gun" has a 1.4 mm nozzle and puts out an incredible amount of product in a spray pattern nearly 12" across held 8 inches form a panel. I've had this gun for over 20 years, and have sprayed literally 10's of thousands of liters of paint through it. However it would be a very poor gun for doing bursts.

If I was to change the nozzle on this gun to 1.8 mm I'd be able to spray peanut butter through it.

For most things in guitar building a gun like the last one on your list should do the trick, but without actually having tried one, it's only my best guess.

Almost all spraying problems can be overcome by modifying the viscosity of the spray material, the air pressure to the gun, the fluid adjustment, or spray technique.

Of course the most efficient method is to keep the product constant and having a gun for each specific purpose. Viable option when you make a living at it, but much more cost effective if you only do occasional spraying to have one gun and learn what changing these things will do for you. If you read through my tutorials, you should get a better idea.
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Nick
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Post by Nick » Wed Sep 09, 2009 5:31 am

Thanks for the insight Allen, I hadn't realized/known that modern HVLP guns could be used with conventional compressors! I've just stuck with the older system and never really looked too closely at these as I used to mean a whole new setup. Apologies jjh for putting you crook. What was that saying about the old dogs and new tricks? :lol: :wink:
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Allen
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Post by Allen » Wed Sep 09, 2009 5:54 am

The new HVLP guns that I've used don't spray like the old, turbine powered ones. In all honesty, those are true HVLP, but are a lot more difficult to use. The new ones that use regular compressed air aren't anywhere near as efficient as the turbine models, but are much easier to use.

The only advantage to HVLP use is in less product going up the exhaust stack. A big factor with production work, as even 10% savings here adds up to significant savings on the bottom line, not to mention the environment.

However all these savings are only as good as the applicator. Totally goes out the window with poor technique. Something that is totally overlooked by everyone when they start sprooking the benefits of HVLP.
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Post by kiwigeo » Thu Sep 10, 2009 11:29 am

Nick O wrote: If you intend doing a few bursts then spend a few more reddies and get a good quality gun.
New Zealand..the only country where they make the $100 bills red so girls can see you pulling them out of your wallet from across the other side of the bar.

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Nick
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Post by Nick » Thu Sep 10, 2009 1:15 pm

kiwigeo wrote:
Nick O wrote: If you intend doing a few bursts then spend a few more reddies and get a good quality gun.
New Zealand..the only country where they make the $100 bills red so girls can see you pulling them out of your wallet from across the other side of the bar.
Listen to the man....He knows what he's talking about. :wink: :lol:
Although I don't bother with the wallet Martin, I keep 'em rolled with a rubber band around the outside. Works everytime.
Image

Normal "on topic" posting will now resume.
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Post by kiwigeo » Sat Sep 12, 2009 10:46 am

Dressed for one of your Chem 101 Tutes?

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John Steele
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Post by John Steele » Sat Sep 12, 2009 11:10 am

Image

And I always thought it was my great smile.....
:lol: :lol: :lol:

Sorry had to see that pic one more time...
J
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