Back to Archive

With Chris Curwen & Expert Auto Painter - Dave Gage (Car Cosmetics)

May 22nd, 2007 - Issue 04

In this issue:

•  Air Compressors, including valuable information on Volume, Pressure and Dry Air.

You could buy the most expensive auto paint products available and spend endless weeks preparing your vehicle's surface to perfection.

You may even be using the best spray paint gun available BUT ultimately end up ruining your entire paint job because of relying on an inefficient air compressor or using a holding tank loaded with moisture and oil residue.

It is vital to the success of your project that you have access to a clean, dry and controlled source of air pressure.

Even the tiniest particles of water, oil or rust will find their way from the holding tank into a spray gun nozzle unless they are captured along the way.

If these particles accumulate and then leave the spray gun nozzle, you will have blemishes on the surface of your paint job.

If painting is only your hobby, then a small compressor will be sufficient. You will find a decent selection of good compressors in shops in the $300 to $500 range and even large, shop-quality compressors for less than $1000.

Make sure that the compressor you purchase has a compressor rate of 5 horsepower or greater. This will offer you plenty of compressed air without it having to run continually.

The harder a compressor has to work to maintain the proper amount of pressure, the hotter the air supply will become. This will allow moisture to be introduced into the air system through condensation inside the piping.

You need an air compressor that can build up a reserve of compressed air and then shut off for long enough to cool down.

Since the amount of room in your garage or workshop can be limited, an air compressor with a vertical tank would be a sound investment.

Even if your air compressor has wheels, find a permanent location for it and secure it in place. Make sure it has a long enough hose to reach anywhere in the work area that you may need, minimum of 25 feet to paint a car but 50 feet works best along with at least 3/8 inch internal diameter.

You should plan ahead when you're shopping for an air compressor for future tool purchases you may make. In the beginning, you may need only enough air for a spray gun, yet you may later decide that you need a whole host of other air tools.

Since you will probably just be using one at a time in your garage or workshop, determine the air pressure and volume requirements of the air tool that requires the most and then purchase an air compressor that will fill that need.

Now we'd like to cover Volume with you.

You can determine the size of air compressor that will work best for you by comparing the cubic feet per minute (CFM) of air needed with your spray gun and the application of specific paint products to the CFM rating on the compressor you are thinking of using.

If the compressor is able to meet the required CFM, then you will be fine. However, if you need 14 CFM for your spray gun and the 2 horsepower compressor you have has a maximum of 9 CFM then you will have to rent or borrow an air compressor with a CFM rating of 14 or more.

You will also need to make sure your compressor has adequate capacity. For example, a 5 horsepower compressor with a 20-gallon tank supplies enough air for a conventional spray gun, but it may not be able to meet the demands of a HVLP spray gun.

That will cause you to stop, before you have finished applying an entire coat of paint in one sitting, to allow the air supply to catch up. The same 5 horsepower compressor with a 35-gallon tank would eliminate this problem when using a HVLP paint spray gun.

And now Pressure:

Be advised that air hose creates a pressure drop. The longer and smaller the hose, the greater the pressure drop is between the compressor and the paint gun 25 or however many feet away. It can be as much as 10 psi per 25 feet of length with 3/8 inch hose. This is another reason to purchase a better quality compressor.

To make sure you have the right amount of air pressure reaching the tip of your spray gun, hold the trigger on your gun wide open while you adjust the air pressure regulator.

The control gage may show 40 psi while the gun is idle but once you pull the trigger it may drop to 30 or 35 psi.

It is very important that paint is applied at the right psi rating as indicated on the product container label or in the literature that came with the application guide.

Finally, Dry Air:

Compressing air separates water from it, turning humidity into water droplets. For this reason, you will need a good water trap at the compressor outlet.

Having it located about 25 feet from the outlet will give you the best results. You should also place another one at the end of the hose near the paint gun. Your air compressor will have a water outlet on the tank; make sure you drain it regularly.

Ideally, you should place a copper or galvanized pipe running downhill from the compressor towards the water trap. This will help the accumulated moisture in the heated air to move downhill towards it.

The hot air will have time to cool while inside the pipes therefore allowing the moisture in the air to condense into droplets. These droplets would then be captured and held in the trap as a liquid.

You should connect your air compressor to your piping system using a short, flexible air hose. This will allow you to easily disconnect the air compressor from the piping system if you need to relocate it for various jobs.

As mentioned earlier, you don't want any water in the air that you will be using to spray paint with. You don't want any other particles to get in there either.

All air compressors have an air intake port that comes with a replaceable filter. For best results have a few extra filters to hand so you can change them frequently.

The air compressor will likely be in the garage area where you will be doing most of the painting. This means the compressor is taking in air that is filled with paint overspray.

If you check the air compressor's filter after you complete your paint job, you will be surprised at what you find. For the best results, place the compressor where it will take in the cleanest air it can.

That's all we have for you this week. Thank you very much for reading. We both hope this issue has been of valued interest to you.

Next week we'll make a start on Surface Prep.

Happy Painting!

Chris Curwen & Dave Gage
www.LearnAutoPainting.com