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Old 03-27-2013, 10:50 PM
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15 gal air compressor for priming engine bay

Hi,

I am new to the forums. I read some articles and threads here and I am amazed with the information provided.

I have been reading for hours. Thanks everyone for sharing their knowledge.


I have a project car that I am working in my small garage. I am redoing the engine bay. I stripped it to bare metal replaced some panels, new wheel tubs, lots of welding etc.

Can I spray Epoxy primer, 2k high build primer and sealer with a 15 gallon air compressor ?

It says 5hp , 6cfm at 90psi on the compressor but I think those numbers are inflated.

I was thinking it is possible since the engine bay is fairly smaller surface than the exterior panels. And I can do it in sections.

My real concern is not to get water vapor in the gun. Any recommendations to prevent that ? Draining the tank, adding oil/water separator.

Would it help to let the air compressor cool down after it is filled ?

Any recommendations are appreciated.

Thanks....

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Old 03-28-2013, 06:34 AM
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You are correct in assuming that your compressor ratings are inflated...most manufacturer's of smaller compressor do embellish not only the CFM output but, if your compressor is plugged into a 110 circuit...I highly doubt your getting the advertised 5 HP out of your electric motor. If in fact you are running a 110 Volt outlet to run your motor, your HP rating would be closer to 2 HP.

You have a 15 gallon holding tank, your compressor is rated at 6 CFM at 90 PSI, which means that at 30 or 40 PSI, your maximum CFM output would be (and this is a guess as I don't know other specifications of your compressor) about 9 CFM...that is maximum. Your average spray gun uses about 10 to 12 CFM...so to answer your question with a question, how small a surface area can you break your engine bay up into to prime? If you broke it up into 6 pieces, rad support, driver's side inner fender, driver's side wheel well, firewall, passenger's side inner fender and passenger side inner wheel well...you would still be pushing your compressors, capability to prime your project in sections. The compressor would be running constantly and as you mentioned, it would create moisture from the heat build up...even on smaller areas like your inner wheel wells. You can add a oil/water separator...it would help...for a little while but, as you continue to prime, water will get through your gun and onto your painted surface. It would help to let the compressor cool down when your holding tank is depleted and your compressor has run for a while...but if you plan on priming all sections of your engine bay in one day and not get any moisture in your primer...you will in all likelihood be there all day and still be at risk of getting moisture in your primer.

One suggestion I could make to make life easier on you, (not so much for your compressor) would be to get a larger holding tank...you have 15 gallons now, if you where to add another 45 gallons, you may be able to get through priming 1 of 6 sections without your compressor being able to spit water out through your gun...band aid on a broken leg scenario...but other than accessing a larger compressor, that's all I could with clear conscience recommend. In fact, if you really like your car and you want to do a good job without a probability of having moisture in your primer...get a compressor that puts out more CFM, has a larger holding capacity....and if at all possible (don't know if you have 230 connection your garage but) an electric drive motor that is truer to the rated 5 HP you have now.

Sorry to be the bearer of not so great news but, you asked the questions and I look at it as though I'm an honest salesperson at a compressor store...I would recommend a larger compressor at the time you purchased it with what you want to use it for now.

Hope this helps.

Ray
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Old 03-28-2013, 08:00 AM
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The problem with adding another 45 gallons to that little compressor is that it would take forever for it to fill up! Even if someone is willing to wait all that extra time to charge the the tank it would GREATLY exceed the compressor's duty cycle, which is going to be quite low anyway on that unit, it for sure would result in an overheated pump! Unfortunately adding a bigger tank does work even on larger compressors and the problems it would crate are even greater on one this small.
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Old 03-28-2013, 09:04 AM
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Thanks for the great responses.

I am going to look at couple places for renting an air compressor.
If I can't find one for the job than I will take it to a shop and get it done. I thought a 15 gallon would at least do the engine bay in sections.


These guys rent 5hp gas air compressor. Does electric or gas air compressor matter for painting ?

Rental Equipment in Windsor supplied by Sunrise Equipment Rental & Sales
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Old 03-28-2013, 09:31 AM
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Quote:
Originally Posted by oldred View Post
The problem with adding another 45 gallons to that little compressor is that it would take forever for it to fill up! Even if someone is willing to wait all that extra time to charge the the tank it would GREATLY exceed the compressor's duty cycle, which is going to be quite low anyway on that unit, it for sure would result in an overheated pump! Unfortunately adding a bigger tank does work even on larger compressors and the problems it would crate are even greater on one this small.
I appreciate what your saying and understand that filling another tank 3 times the size would tax the small compressor and I did mention that it was a band aid for a broken leg scenario. I was trying to give the OP some direction in which he could use is compressor and prime small areas...I'm thankful that he's looking at other solutions.

Ray
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Old 03-28-2013, 09:39 AM
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Quote:
Originally Posted by Serkan View Post
Thanks for the great responses.

I am going to look at couple places for renting an air compressor.
If I can't find one for the job than I will take it to a shop and get it done. I thought a 15 gallon would at least do the engine bay in sections.


These guys rent 5hp gas air compressor. Does electric or gas air compressor matter for painting ?

Rental Equipment in Windsor supplied by Sunrise Equipment Rental & Sales
What matters more than anything is the CFM output of the compressor and how clean the air coming out of the compressor is...if your looking at a rental unit make sure that it has more CFM capacity than what your paint gun puts out. If your paint gun requires 12 CFM at 30 PSI, the compressor should put out 18 CFM minimum at the same PSI. The horse power rating doesn't mean a thing...Think of it like drag racing, if I'm running a car with 1000 HP and I'm racing a guy with 500 HP, all that matters is who gets to the finish line first...with a compressor the horsepower rating is what moves the pistons in the compressor head, it's the compressor head output that your interested in.

Gas, Diesel or Electric, what matters is CFM output and air quality.

Ray
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Old 03-28-2013, 09:44 AM
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My reply was not intended to chide anyone, sorry if it came across that way. Rather it was simply pointing out that attempting to use a larger tank to make up for a lack of CFM for spray painting does not work because any gains would be cancelled by the proportionaly longer recharge time, also that in this case it could result in a damaged compressor from overheating.
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Old 03-28-2013, 09:48 AM
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Quote:
Originally Posted by oldred View Post
My reply was not intended to chide anyone, sorry if it came across that way. Rather it was simply pointing out that attempting to use a larger tank to make up for a lack of CFM for spray painting does not work because any gains would be cancelled by the proportionaly longer recharge time, also that in this case it could result in a damaged compressor from overheating.
Nor Chide taken...I think we are on the same page and what's important is that the OP understands...I guess the simple answer would be....No the compressor is to small....LOL.

Ray
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Old 03-28-2013, 04:59 PM
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Old Red is so right, the size of the tank makes no difference. It takes x minutes to fill it, once it is drained down to the kick on point of the compressor it is going to run continuous until it either gets the pressure back up to shut off, or run until you quit using air, then run x additional minutes until the tank is back to full pressure.


If the tool says it used 9 cfm @ 90 psi and your compressor will only make 6cfm @ 90 it will never keep up no matter how big the tank within reason. Maybe a 5000 gallon tank would last long enough for you to do something before the compressor had to run, but it would run a loooong time to fill and refill the huge tank.
The bottom line, when using air sanders and spray guns, the usage is quite substantial and as such takes a substantial air compressor to supply air.


20 amp, 240volt motor is about the smallest that can keep up. Less than that you can get by, by doing panels and waiting and dealing with moisture issues.
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Old 03-28-2013, 05:15 PM
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I agree with what both of you are saying...the compressor will not keep up, there is no question about that and I also agree that the larger the holding tank the longer the compressor would need to run to fill it. A 15 gallon tank at 90 PSI would last about 30 seconds tops with the compressor running. With an additional 45 gallons of holding capacity, with the compressor running would give the user about 2 to 3 minutes of spray gun usage...that should be enough time to put a coat of primer on an engine bay if done in 6 separate pieces (as I mentioned in my original post).

"One suggestion I could make to make life easier on you, (not so much for your compressor) would be to get a larger holding tank...you have 15 gallons now, if you where to add another 45 gallons, you may be able to get through priming 1 of 6 sections without your compressor being able to spit water out through your gun...band aid on a broken leg scenario"

If the OP had a compressor that could handle the job, I'm sure he could put a coat of primer on the engine bay in less than 12 to 15 minutes. I just happy the OP understands that he needs a larger compressor.

Ray
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Old 03-28-2013, 09:32 PM
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I'm not going to speak in hypotheticals and theoreticals here, but I'll speak from practical experience with what I've actually done. I have a 60 gallon 3.7 hp 13 CFM compressor now, but before that I had a Crapsman 5 impossible HP from 110V 20 gallon. I ran a whip from it to a water filter/regulator on my wall, and that's where my main line tied in. I could easily spray an engine bay with my 20 gallon. In fact, when I moved out here to Michigan, before I got the 220 ran in my shop for my big compressor, I was spraying with my little 20 gallon and it does just fine for small things like an engine bay. I know yours has 5 gallon less capacity, but take this for what you will.

Something else to chew on. I took my engine bay down to bare metal, primed with Sikkens 1K wash primer (etching primer in a rattle can) and sprayed two coats of Bill Hirsch chassis black (also in a can). The cans have the tip that sprays the pattern so it lays on nice. This I did in 2009. It looked great then. And still looks as good today as it did then. There's only one part of my car that I haven't had to continually sand down and respray because of rust popping through throughout the course of my restoration, and it's been the engine bay.
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Old 03-29-2013, 06:11 AM
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The little 15 gallon compressor should be capable of doing something like an engine compartment but of course it depends a lot on his other equipment. If using a conventional paint gun and the lowest air pressure that will still do a good job plus working in sections then it should work just fine, people have even painted whole cars with little compressors. However if the spray gun is a HVLP type and/or the whole compartment is sprayed in one operation then this is going to be a very frustrating endeavor! The trick to using a small compressor is to accept it for what it is and properly MANAGING what air is available because nothing can be done to make the compressor put out more air than it does already. Instead of trying to squeeze more air out of a too-small compressor concentrate on spraying the most paint practical with as little air as possible while still maintaining good results.


widetrack, I see your point and I understand that if 15 gallons gave 30 seconds of spray time (it wouldn't even be that long with only 15 gallons!) that an additional 45 gallons would add more time (another 90 seconds theoretically) but that's only 2 minutes of spray time, which is probably too optimistic, and then the problems would begin. While in theory it would take 4 times as long to fill a tank 4 times as big in reality it would not work that way because as the compressor heats up efficiency falls off rapidly. On a setup such as this by the time the tank was half filled the pump would probably only be able to manage 50% or maybe even less of it's original CFM rating, this problem will only compound itself the longer the compressor runs. If this is one of the little oil-less type compressors then filling that tank a couple of times or so without long cooling periods will most likely result in destruction of the pump. I have seen it tried many, many times and adding significant extra tank capacity will almost always result in more frustration rather than solving the problem.
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Old 03-29-2013, 06:24 AM
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I had hooked up a 30 gallon tank to my 20 gallon compressor a few years back and I think it literally bought me an extra 10 seconds of spray time because once the pressure decreases the compressor kicks back on again. And it didn't take any time at all for the pressure to decrease. However, it took it forever to build the pressure back up.
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Old 03-29-2013, 07:41 PM
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I was laughing at the comment about using a large tank to pre fill before starting a painting project as thats actually what I did a number of years ago. Mind you it wasn't a vehicle but was pouring through gallon after gallon of tremclad rust paint onto grain bin hopper bottoms. I used a 500 gallon propane tank on wheels and pressured it up to 175 psi with a true 5 horse two stage compressor. That took an hour from empty to full to fill that tank size but the compressor is made to run fairly continuously and I've done that a lot in filling the tank for other air needs and the compressor never has missed a beat all these years. I was using a one gallon if not more canister type painter that would have used fairly low air volume and a long set of hoses off of that to the gun.
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Old 03-30-2013, 09:49 PM
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Wow thanks for all the replies. You guys are a great bunch in this forum.

I don't know how you guys treat outsiders but the car I am working on is actually an import

I will be calling the rental places after the holiday. Meanwhile I asked a friend and he has a 25 gallon compressor same brand as mine with inflated numbers

What if I were to use both compressor at the same time, switch the hose right away when pressure drops on one .

Here is a pic of the engine bay. I still have to sand down the frame rails and strut tower skirts to bare metal. I am not touching the firewall, it is scuffed up.

So there are only 2 sides and inner wells which aren't completely stripped.

This is a 2004 subaru impreza engine bay.

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