Compressor Setup for Spraying

The Achilles' Heel of Luthiery
Post Reply
Darryl Young
Posts: 1668
Joined: Fri Jul 30, 2010 6:44 pm
Location: Arkansas

Compressor Setup for Spraying

Post by Darryl Young »

I bought a used compressor this weekend. It's a 220V, 15Amp motor with a 30 gal tank. It puts out plenty of air for my purposes.

Yesterday, I bought a filter to remove moisture from the compressed air. I know this is mandatory for spraying. I also have a small moisture trap that will mount under the gun just in case anything gets by the first filter so I think I will be ok in the moisture removal department.

An issue I need feedback on is removing the heat from the compressed air. I understand that air is heated when it is compressed and I know there are air coolers to remove some/most of this heat. Do I need one of these to lower the air temperature to successfully spray waterbase finish? I know the KTM-SV I will be using (arrived yesterday) has a tendency to dry before it hits the surface (createing a pebbly surface) so you need to hold the gun relatively close and spray fairly "wet" (but not so wet it will run). I'm wondering if heated air will make this problem more apparent.

And finally, can I get by with a 50ft air hose or do I need to try to plan a setup where I can use a shorter hose? How short do I need to go?......25ft?

Thanks for your input.
Last edited by Darryl Young on Tue Sep 21, 2010 1:27 pm, edited 1 time in total.
Slacker......
Freeman

Re: Compressor Setup for Spraying

Post by Freeman »

Darryl, that is pretty much what I use. I picked up a little home air compressor at a garage sale - don't know the tank size but it isn't very big. Added a water trap and regulator, I don't have anything to remove heat (nor have I noticed any problems) I have about 25 feet of hose (my compressor is right next to my work bench but I would like to locate it more distant because of the noise). When I'm spraying I simply start it and let it run for a few minutes until the tank is full and up to pressure - it will come on and off but maintains the volume and pressure just fine.

I use a little detail or "jam" gun (a body shop would use it to shoot door jams or other small areas) - I may have bought it at Grizzly but any good auto paint store should be able to sell you one. There are many other kind of guns - but this has worked well for me. I've also used my little air brush with this compressor to do sunbursting - had to make up some hose adaptors to get to the correct size for the fitting on the air brush.

One potential problem with many guns that are designed for solvent finishes is that they will rust if you simply clean them with hot soapy water (which works fine for waterbased finishes like KTM-9). I clean the gun in the sink, then fill the cup with denatured alcohol and shoot some out of the nozzle, leaving the rest in the cup until my next session. I fire up the compressor, shoot a tiny bit of alcohol into a rag, then do a fine mist onto the previous coat on the instrument. Dump the alcohol out and fill the cup - during that time the surface of the old finish will get slightly tacky - that makes the next coat melt into the old one.

I don't remember what pressure I finally ended up with - somewhere between 35 and 40 psi I think (I know I don't have to change it when I air up car tires). You will want to experiment with your gun - adjust the mixture needle and pattern to get a pattern you like. I tend to shoot a little on the wet side and once in a while do get runs - I like that better than getting a lot of orange peel. For the air brush I really turned the pressure down (10 or 15 psi I think).

My humble experience with KTM-9 is that diluting 1:1 makes a good wash coat, full strength for all building. I've heard that you can dilute that last couple of coats and shoot pretty wet to bring out gloss but I've been fine with standard color sanding and buffing.

Dan Earlewine's book has some pretty good information on basic home spraying as well as all the other steps of instrument finish.
Darryl Young
Posts: 1668
Joined: Fri Jul 30, 2010 6:44 pm
Location: Arkansas

Re: Compressor Setup for Spraying

Post by Darryl Young »

Thanks Freeman.

I have already bought a gun. I bought a Walcom EGO which is a fairly nice gun. This is the same gun Rolf Gerhardt uses to spray KTM-SV. He uses a 1.2mm needle so that is what I bought. This gun has an all stainless steel fluid passage so is not supposed to rust......but the alcohol idea sounds like it would work well for cleaning in-between coats.

I will call KTM and ask if an air cooler is needed.

BTW, KTM-SV is a spar varnish so it's a different product than KTM-9 which is an acrylic lacquer. Both are water based finishes.

Edit to add: I just got off the phone with Grafted Coatings. They said they have a cooler on their compressor they use to spray in-house but they said that by far most of their customers don't have an air cooler and they think it works fine. I will test it out on a test piece anyhow so I will test without a cooler.
Slacker......
Ken C

Re: Compressor Setup for Spraying

Post by Ken C »

Darryl,

You shouldn't have to cool the air unless you have high relative humidity and can't get the moisture out of your air. You may then need to cool the air to get all the moisture out of the line. I run 25' of hose to a moisture trap, then another 25' to a the gun, where I also have a small disposible filter. You can always pick up some retarder to slow the curing process (I assume Grafted Coatings makes a retarder).

Practice on a few boards until you get your fan size and fluid controls dialed in where you want them. I try to shoot my EM6000 at about 6" to 8". Any further, I have to spray too heavy of a coat for it to go on wet, and any closer results in excess overspray. Also make sure your pressure at the gun with the trigger pulled is what the gun manufacturer recommends. Even if you have a regulator at the gun as I do, set your pressure at the tank with the regulator at the gun wide open and only make very minor adjustments at the gun.

How often do you have to wait between coats for the SV? The joy of lacquer is that it can be shot every 45 to 60 minutes. I'd hate to wait a couple of hours between applications. With lacquer, I can shoot three or four coats in an evening after work.

Ken
Darryl Young
Posts: 1668
Joined: Fri Jul 30, 2010 6:44 pm
Location: Arkansas

Re: Compressor Setup for Spraying

Post by Darryl Young »

Ken C wrote: How often do you have to wait between coats for the SV? The joy of lacquer is that it can be shot every 45 to 60 minutes. I'd hate to wait a couple of hours between applications. With lacquer, I can shoot three or four coats in an evening after work. Ken
The folks I've talked with (Rolf Gerhardt and Randy Muth) and another gentlemen, Tom Bazzolo (whose schedule was sent to me along with Rolf's) say they wait about 1 hour between coats. Here is a clip from the spray schedule Rolf sent me:

Spray coats 1 hour apart (should be dry to touch in 20 minutes, dried through in 40 minutes, dried hard in 60 minutes).

They spray 6 coats the first day, level sand and spray 6 more coats the second day. two days later (day 4) level sand (before the finish gets to it's final hardness where it takes more effort to sand), and buff out the following weekend (day 6 or 7). One thing that appeals to me is completing the finish process in roughly 8 days. In fact, if you've done it a time or two you could pore fill 3 times with zpoxy and go through the entire finish process in 10 days.

I have the impression that it is best to add coats in the manner described above as this product cures quickly compared to lacquer so burn-in is best when applied before the previous coat cures too much. The level sanding on the second day (400g - 600g) also provides a mechanical bond for the first coat of the second day. You can also spray a mist of alcohol to soften the previous day's finish slightly before starting to spray on the second day. They claim the product levels well enough they use 1,000g to level sand on day 4 and work up through the grits from there.

It will be the first "real" finish I've attempted though I have used polyurethane, Danish oil, etc. previously. I'm looking forward to trying it. I like that it is more protective (harder) than lacquer yet is still very flexible. The speed of the finish process is a bonus.

Edit to add that Randy Muth sometimes waits 1 1/2 hours between coats while the others re-coat once per hour.
Last edited by Darryl Young on Fri Sep 24, 2010 9:06 am, edited 1 time in total.
Slacker......
kredwards

Re: Compressor Setup for Spraying

Post by kredwards »

I was asking the same type of questions of a friend. He tells me that expensive guns heat the air used to spray. He says that the problem with normal guns, and aerosol cans for that matter, is the spray cools down as it leaves the nozzle. This is to be expected just from expansion. The cool spray causes problems getting a good smooth coat. StewMac actually recommends placing aerosol cans of lacquer in a warm water bath prior to using and keeping watch on the can temperature during use. If the can starts getting cool, switch to one that is warm.

I don't know how much of a problem any of this is since I have no experience spraying lacquer. I will tell you in a week or so.

Kent
Darryl Young
Posts: 1668
Joined: Fri Jul 30, 2010 6:44 pm
Location: Arkansas

Re: Compressor Setup for Spraying

Post by Darryl Young »

Thanks for that response Kent. I wasn't taking into consideration the air cooling when dropping from high pressure to low pressure (just like it heats from being compressed from low pressure to high pressure). Thanks for the reminder.
Slacker......
Post Reply