Greetings all, glad I found this site.
I've been considering painting my old motorhome, but after reading a lot of the info on this site that may not happen. Why? Air compressor!
Let me give you the details. I have a Porter Cable 6hp, 20gal tank that rates at 8.3 scfm @ 40psi and 5.8 scfm @90psi. The compressor was about the last thing on my list of things needed to make this paint job happen. Heck, I have one so check out the specs on it later. Well, I checked Maybe not all hope is lost.
Let me run this by you. My first choice for a paint gun (cost wise) is a FinishLine 4 FLG-670 and second Finex FX3000. Want to shoot acrylic enamel paint. Since this is a motorhome (on a 73 Dodge cutaway 19') and way too big of a job for complete coat at one time, at least for me, I'm going to remove all doors, hood and hatches and paint separate from the body of the coach. As for the coach, I believe it can be done in five sections, the roof then half of the sides at a time. Do two coats and maybe a clear before moving from one section to the next. Yes, a lot or recovering and masking, but this is a one time job and I have the time to do it. No rush.
Now, back to the real issue, the compressor. Since each part, section will not take an extended time to paint, I'm wondering if this approach might work with the compressor I already have? Perhaps not with the FinishLine, (my choice) but maybe the Finex?
Looking forward to you input.

Welcome.

Forget the FinishLine gun, it will suck down 13 CFM easily.

I believe the Finex 3000 is similar; HVLP and will take a LOT of air. You might be able to get by with a conventional pressure gun like the Finex 2000 or a RP gun like the GFG-670 DeVilbiss Plus. I carry both of them:

http://www.autobody101.com/store/spray-guns/

I used the Finex 2000 with a 1.4 tip once to do a complete repaint on a car from primer through clear, just to see if I could do it. I was using a small portable compressor rated 9 CFM at 40 PSI and took my time. It worked out ok.

I have both the FX-3000 and the Finishline FLG-4. The FX-3000 consumes 9.5 CFM with maximum of 29 PSI at the inlet. I used the FX-3000 to spray conversion varnish on sets of cabinet doors using a 1.8 HP 30 gallon Kobalt compressor for a couple of years. On some jobs, I'd spray 40 doors/drawer fronts at a time, only pausing to place the wet door on the drying rack and grab the next one. I never had to wait for the compressor to catch up. Granted, a cabinet door is small compared to a truck door or hood, but my gut feeling is that you can use the FX-3000 this way. I'll check the CFM/PSI specs on the compressor and update.

Another thing you could do is add an extra air tank by installing a tee fitting between your main tank and air switch and connecting that to the extra tank. It won't increase the CFM rating, but it will give you extra air volume. Harbor Freight has inexpensive air tanks, $37 for 11 gallon. I'd scour craiglist and other classifieds to see if I could find a dead compressor with a good, large tank.

OK. Math/Engineering time here kids! Bear with me, as this is kind of long.

A 60 gal tank taken from a large compressor is = 8.02 cu. ft.

If that tank is used at 90 psi, you effectively have about 6X the air at normal atmospheric pressure (normal atmospheric pressure is 14.7 psi - but we'll use 15 psi for our rough calculations here).

Remember - you need air out of the tank at 40-50 psi. Let's split the difference and say you need 45 psi due to fittings, line losses, etc. (So, if you provide less than 45 psi you won't get the atomization you need!). So, using a 60 gal. tank gives us 8 cubic feet of 45 psi air to work with (= 90psi in tank - 45 psi min needed).

Since most spray guns are measured in SCFM (standard cubic feet per minute), we need to convert to this unit of measure. SCFM by definition is the amount of air per minute at standard room temperature and normal atmospheric pressure. Air at 45 psi (from the above equation 90 psi - 45 psi) is 3x standard atmospheric pressure (45 psi / 14.7 psi = 3.06). This gives us an effective usable volume of air in the tank at 8 cu ft. x 3.06 = approx. 24 standard cubic feet (SCF).

Now - let's assume the spray gun uses 10 SCFM. This gives us an extra 2 minutes and 24 seconds of air (= 24 SCF in tank / 10 SCFM).

Natemoore suggests a small but cheap 10 gallon tank at 90 psi. This is only 1.34 cubic feet! Using the above formula to find SCF, 3 x 1.34 = 4 cubic feet at standard conditions. Using the same spray gun at 10 SCFM, this equates to less than half a minute extra run time (4 SCF / 10 SCFM = 24 seconds).

Is this enough extra to do the side or roof of a camper? IMHO, Probably not. Would the 60 gallon tank work? Depends on the size of the camper. Not sure you want to split the sides or roof in 2 sections each (6 total) as you would likely have blending/color match issues.

I leave it to you to decide if either scenario will work for you. But I hope this has been educational for everyone!

1MechEng wrote:OK. Math/Engineering time here kids! Bear with me, as this is kind of long.

A 60 gal tank taken from a large compressor is = 8.02 cu. ft.

If that tank is used at 90 psi, you effectively have about 6X the air at normal atmospheric pressure (normal atmospheric pressure is 14.7 psi - but we'll use 15 psi for our rough calculations here).

Remember - you need air out of the tank at 40-50 psi. Let's split the difference and say you need 45 psi due to fittings, line losses, etc. (So, if you provide less than 45 psi you won't get the atomization you need!). So, using a 60 gal. tank gives us 8 cubic feet of 45 psi air to work with (= 90psi in tank - 45 psi min needed).

Since most spray guns are measured in SCFM (standard cubic feet per minute), we need to convert to this unit of measure. SCFM by definition is the amount of air per minute at standard room temperature and normal atmospheric pressure. Air at 45 psi (from the above equation 90 psi - 45 psi) is 3x standard atmospheric pressure (45 psi / 14.7 psi = 3.06). This gives us an effective usable volume of air in the tank at 8 cu ft. x 3.06 = approx. 24 standard cubic feet (SCF).

Now - let's assume the spray gun uses 10 SCFM. This gives us an extra 2 minutes and 24 seconds of air (= 24 SCF in tank / 10 SCFM).

Natemoore suggests a small but cheap 10 gallon tank at 90 psi. This is only 1.34 cubic feet! Using the above formula to find SCF, 3 x 1.34 = 4 cubic feet at standard conditions. Using the same spray gun at 10 SCFM, this equates to less than half a minute extra run time (4 SCF / 10 SCFM = 24 seconds).

Is this enough extra to do the side or roof of a camper? IMHO, Probably not. Would the 60 gallon tank work? Depends on the size of the camper. Not sure you want to split the sides or roof in 2 sections each (6 total) as you would likely have blending/color match issues.

I leave it to you to decide if either scenario will work for you. But I hope this has been educational for everyone!

A commuter airliner was enroute from Warsaw to Krakow when the pilot pointed out the picturesque Vistula River on their left. Passengers stood up and leaned over to peer out the windows. A few minutes later, the pilot points out the snow topped Carpathian Mountains to their right. Again, passengers stood up and leaned over to gaze at the sight. The airliner immediately spun out of control, broke apart, and crashed to the ground. There were too many poles in the right half plane. Sorry, esoteric engineering joke from long ago.

You didn't take into account the air that would be added back in by the compressor, did you, or did I miss that? Seems to me in your scenario, you are only calculating how long the air already in the tank would last, disregarding the 5 or 6 CFM that would be added to it while it was bleeding down. Your formula should really be a constant plus a time dependent variable, i.e., at some time after squeezing the trigger, the pressure and volume would drop below what the spray gun requires. If this time is is less than the time needed to do a coat on a hood or a door, then there could be problems, but choosing the right reducer would buy some time to allow the compressor to catch up.

BTW, my 1.8 HP 30 gallon Kobalt has the following specs: 5.7 CFM @ 90 PSI and 7 CFM at 40 PSI. My FX-3000 consumes 9.5 CFM with the inlet gauge set at 25 PSI. Based on my experience, without doing LaPlace transforms and stuff, I think I could spray individual doors or a hood before it petered out.

In fact, I just went out to the shop, shut off the tank of my 60 gallon compressor (my two are set up in tandem), set my line regulator at 90 PSI, my gun regulator at 25 PSI, and pull the trigger/set a stop watch. It was able to hold the 25 PSI for over 5 minutes, making a couple of adjustments to the gun regulator during that time. When I stopped the experiment, the line regulator read 70 PSI. Held my hand over the air nozzle the whole time and certainly didn't feel any difference in air pressure. So, how big of a panel can one paint with 5 minutes of continuous spraying time?

Also, I readjusted the kick-on and kick-off pressures to 90 and 120 respectively. My compressor can go as high as 155 PSI. So, depending on the OP's maximum tank pressure and the adjustability of his pressure switch, he will probably be able to use the FX-3000 with no problems if he breaks it down into reasonable sizes.

And who cares how bad the roof looks, unless you're talking about the roof of the truck, not the camper shell.

Thanks for all the replies. Giving this more thought it's best that I look for a used compressor that can do the job right. Spending lots of money on all other supplies to do what I hope will be an above average paint job, makes no sense to short change on the compressor requirements. And you're right nate, who cares about the roof of an RV. I'm still planning on doing sections at a time and the roof will be the first large are, that will be for practice.
Now I'll move to paint and boby with questions. Thanks again all.

Oldvanman wrote:Thanks for all the replies. Giving this more thought it's best that I look for a used compressor that can do the job right. Spending lots of money on all other supplies to do what I hope will be an above average paint job, makes no sense to short change on the compressor requirements. And you're right nate, who cares about the roof of an RV. I'm still planning on doing sections at a time and the roof will be the first large are, that will be for practice.
Now I'll move to paint and boby with questions. Thanks again all.

You don't have to go out and buy a 5 HP 60 gallon compressor. Figure out what additional CFMs you need, buy a little, used compressor that will be your "booster," then connect them both to the main line...or you could borrow a compressor from a friend and do the same.

Nate, just so I understand, I can take two compressors and tee them together and get the scfm's I need?

Older thread: viewtopic.php?f=3&t=21558&p=165090&hilit=joining+two+compressors#p165090

Oldvanman wrote:Nate, just so I understand, I can take two compressors and tee them together and get the scfm's I need?

Yes. As the other thread pointed out, you may have to adjust the pressure switches to ensure they turn on and off at approximately the same PSI. Here's a youtube video that helped me understand the various switches used.

https://www.youtube.com/watch?v=zh_hfD3rCA0

On my two compressors, one has the Square D brand and the other has the "furnace" type (I think that's what he's saying in the video). Neither of these have a differential adjustment, which is the range between the cut-in pressure and cut-out pressure. Luckily, both have a differential of 30 psi, so all I had to do was adjust the range adjustment nut. I adjusted my little Kobalt 1.8 HP/30 gallon compressor to match my larger 3 hp/60 gal one: cut-in at 90 psi, cut-out at 120 psi (30 psi differential). If there is any "tamper proof" hot glue on the adjustment nut, just scrape it off. I needed a set of tamper proof torx screwdrivers on one of them. Harbor Freight to the rescue.