Troubleshooting Abrasive Media Flow Issues in Siphon Blast Cabinets

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Why is my sandblaster not picking up sand? It has happened to the best of us. You’re excited to proceed with the next big step in your project. You’re ready to blast that critical part. You fire up the air compressor, pull the trigger on the blast gun, and the results are disappointing. Little to no blast media comes out.

Poor media flow in a siphon style sandblast cabinet can be incredibly frustrating. That’s why we’ve put together the ultimate guide to help troubleshoot and fix blast cabinet abrasive media flow issues. Before we get started, let’s make sure we’re on the same page regarding what type of blasting we’re talking about.

There are actually different types of abrasive blasting. Two main types are direct pressure (using a pressurized vessel) and the simple siphon blasting setup. In this article we’ll discuss how to troubleshoot flow issues in siphon blast equipment, since that is what most affordable blasting equipment is.

An image of a blast cabinet with a sad frown giving a thumbs down gesture. Text reads, "Troubleshooting Poor Media Flow in Siphon Blast Cabinets."

How a Siphon Style Blast Gun Works

A siphon, or suction, style cabinet typically has two different hoses connected to the blast gun: a compressed air line and a suction tube for the abrasive media.

Compressed air flows into the blast gun and then through a narrow air jet. This creates a vacuum that draws media up the separate siphon hose and into the blast gun. From there, the compressed air blows the air and media mixture through a blast nozzle and out the gun.

This simple design is extremely common in abrasive blast cabinets but it does allow a few opportunities for trouble. Anything that impedes the flow of compressed air or abrasive can result in subpar performance. There are numerous factors at play. Don’t worry. We’ve got you covered.

We put together a list of common issues to help troubleshoot poor media flow in a suction blast cabinet. We recommend reading through this post in its entirety before heading to your blast cabinet.

1. Check the Media Hose for Damage or Obstruction

Examine the entire blast media siphon tube and hose for any issues. How detailed you want to be with this inspection is up to you. The best way to do this is to completely remove the suction hose, empty it and look and feel for issues. Look for holes or other obvious damage. Abrasive blast media will slowly chew away at your siphon tube and suction hose from the inside over time. Hoses can rub and chafe against hard corners and more. Replace the hose or tube if you find any damage.

Make sure there are no large particles (debris or large blast media particles) blocking the siphon hose or pickup tube. If necessary, disconnect the line, empty it and check for damage or blockage. If your siphon tube has an air inlet tube ensure there is nothing blocking the flow. If your siphon tube is a tube inside a tube, with a small gap between, ensure nothing is blocking the gap between the tubes.

Check your hose routing. Make sure there are no kinks or sharp bends that might impede flow. If you’re using a foot pedal or a metering valve setup, make sure your hose run is the right length. Make turns with gentle bends using a little extra length. You don’t want a ton of excess and you don’t want to pull the hose tight at sharp angles with no extra slack. Keep the run as short as possible without making it too short.

Observe the siphon hose while blasting to ensure it isn’t collapsing under vacuum. If you’re using cheap hose, the wrong type or worn hose it can sometimes collapse internally during use. This leads to surging or impaired flow. Replace the blast media hose if you find any problems.

2. Check Blast Gun for Blockage and Wear

In step 1 you may end up removing and inspecting the blast tube and hose. That is the perfect opportunity to disassemble and inspect the blast gun. You’re halfway to removing it at this point anyway. Make sure you’ve turned off or removed the compressed air supply and then remove the gun. You may not have to remove it from the cabinet, but removal makes disassembly and inspection easier.

Start by removing the compressed air hose and media hose. Next, remove the nut holding the nozzle tip and remove the blast nozzle. Inspect the nozzle tip for obvious issues. We go into further detail in item #3 below. Look for obvious blockages, wear or damage in the gun and nozzle.

Look for foreign objects inside the blast gun. Debris inside a blast cabinet isn’t anything unusual. Bits of paint, rust, corrosion and dirt are blasted away and end up settling back in the blast media at the bottom of your cabinet. Over the years we’ve found pieces of metal, screws, decals, tape, rubber gloves and other foreign objects in blast cabinets. Large debris can get hung up in the siphon tube, blast hose or metering valve where they’ll block or restrict flow.

Be vigilant and don’t just focus on large debris. It’s the small stuff that usually ends up getting you. Debris small enough to make its way up the siphon tube and blast hose may not cause problems until sucked into the gun.

It doesn’t take something very large to cause issues. A small piece of rubber glove, cabinet sealant or other particle to get sucked up inside the blast gun itself where it can block the internal passages or the nozzle. Even a larger than normal blast media particle can create a restriction. Humidity due to condensation can also cause clumping of media inside the blast gun.

Consider Disassembling Your Blast Gun Further for Inspection

Many issues with your blast gun will be obvious. You’ll remove the nozzle and find a foreign object inside. Other problems aren’t always evident at first glance. Breaking a blast gun down on a workbench can sometimes reveal issues that weren’t obvious. Every gun is different. Some designs allow more disassembly than others.

You may be able remove the internal nozzle or jet orifice area. Others do not. Some guns allow you to adjust the gap between the jet orifice and nozzle which can affect flow. Adjustments may be necessary.

After you’ve got your gun disassembled as far as you feel comfortable with, check for blockages where the abrasive media flows. You can try using a thin piece of stiff wire, nail, or other material to probe inside orifices and clear any blockages.

Inspect for obviously worn or damaged parts. The blast nozzle tip, and internals all wear out over time due to the abrasive flowing through or around them.

On many of the trigger style guns if you remove the nozzle tip and look down the barrel of the blast gun you’ll see a tapered air jet with an opening. Abrasive media flows around the outside of this nozzle. Over time it can eventually wear a hole in it from the outside in. On some triggered guns this jet is removable which may aid in inspection. At the very least, ensure it isn’t clogged or damaged.

Don’t forget to consider the flow of compressed air through the gun. The Harbor Freight style triggered air guns usually have some sort of spring and o-ring setup for the trigger. These seals can wear and leak air. Even if it isn’t your main issue, a leaking or damaged trigger can result in poor air pressure and diminished media flow.

If you suspect a gun issue, or simply want to quickly eliminate the gun as the problem, you can swap out the suspect gun with a known good one. If you’ve got a friend with a good gun ask to borrow theirs. You can also pick up affordable triggered blast guns online at amazon.com and other sites.

3. Check the Blast Nozzle for Damage, Wear, Fit & Compatibility

The blast nozzle tip on your gun is a consumable part. As blast media passes through the nozzle, it slowly chews away at the material and enlarges the opening. A badly worn nozzle can impact media flow and blasting performance.

Most of the blast nozzles on the economy triggered Harbor Freight style guns will be a pink ceramic material. They are affordable but wear out faster and are more easily damaged than other options.

Remove the retaining nut holding your blast nozzle in place. Remove the nozzle. Inspect the nozzle opening for wear and excessive enlargement. Pay close attention to the back side of the nozzle as well. Any chips, gouges or cracks in the nozzle can affect flow. If you notice or suspect issues, replace the nozzle with a new one appropriately sized for your gun and media.

Some nozzles use an o-ring, gasket or other type of seal. Be sure to inspect these if present and replace if damage or wear is noted. Look over the nozzle and area where the nozzle fits into the gun. Although rare, damage to the gun body in this area can potentially cause issues.

Finally, be sure your blast nozzle is the proper size for your blast media. If the opening is too small for your media, flow will be decreased or non existent. Too large of a nozzle diameter can result in poor performance as well.

Are you experiencing new media flow issues after installing a new nozzle tip on your gun? If so make sure your new nozzle is compatible with your gun and fits properly. This is especially true with the triggered, Harbor Freight style guns. Different nozzles are available and not all are made equally.

We’ve ran into an issue once or twice where an aftermarket pink ceramic nozzle fit in the gun but didn’t fit properly. In our case, the ill-fitting nozzles leaked enough to affect flow. We keep a couple old tips around for comparison now.

4. Check Blasting Air Pressure

Generally, most siphon tube style blasting is done in the neighborhood of 60 to 90 PSI. Yes. You’ll sometimes blast higher or lower. The correct air pressure varies from setup to setup. It depends on the type of blast media, the material being blasted, the blast gun and much more.

Our metering valves allow you to blast with better media flow at lower air pressures. Blasting at 40 to 70 PSI isn’t unusual with a cabinet running a metering valve. Generally speaking, too little air pressure will result in reduced or absent abrasive media flow. Blasting at too high of an air pressure can cause problems too.

For troubleshooting purposes, you should be getting some sort of consistent media flow in the 60 to 100 PSI range. This assumes your compressor outputs enough airflow. For more details on that, see item number 9 below.

5. Ensure Compressed Air and Media are Dry

Moisture is your enemy with a blast cabinet. Unfortunately many people just decide air and media are dry. As long as they can’t make a sandcastle with the media and their air hose isn’t shooting water like a garden hose, everything is good. That isn’t reality. Abrasive media and compressed air can look dry but still contain significant amounts of moisture.

Start by using actual abrasive media intended for blasting. Ensure it is dry out of the bag or container. Ensure your compressed air is as dry as possible. One of the main sources of moisture is the air compressor itself.

Compressed air is hot. Compressing air forces the air molecules into a smaller space which generates heat. This hot compressed air contains water in a vapor form. When it enters a cooler area, such as your air lines or blast cabinet, water vapor in the hot air will condense. Warm humid air in a cooler steel blast cabinet and cooler gun provides ample opportunity for condensation. Your media may be dry in the cabinet and then getting wet inside the gun.

If you’re in a humid area, issues due to moisture are more common. There’s already a high degree of humidity present in the ambient air you’re compressing.

Running an undersized compressor compounds the problem. An overworked, undersized compressor is going to run longer and much more frequently. It doesn’t have enough time to rest and cool down between compression cycles. Hotter air means more condensation and more moisture.

If you’re not, you should be running a water separator or some other sort of air dryer. Drain your compressor air tank and water collectors frequently. Sometimes a simple water trap/filter and regulator combo at the cabinet itself is the right solution. Other times you need more. Some sort of air cooler can help if hot compressed air is leading to significant moisture.

6. Check Your Blast Media Level

Too little, or too much, abrasive media can cause issues. You need to have enough abrasive in your blast cabinet to keep your siphon setup supplied with media without overfilling. With a siphon tube or metering valve, somewhere in the neighborhood of 6 to 7 inches of media in the bottom of the cabinet is generally a good rule of thumb. Burying the top of the siphon tube can cause issues with suction.

These recommendations work for our metering valves and most simple economy style siphon tube setups. If you’re unsure, contact your equipment’s manufacturer for their specific recommendations.

7. Check Your Connections

Check your siphon hose connections carefully. Most of the time, these hoses fit very snugly. As a result, many people don’t put hose clamps on the siphon hose, particularly at the gun. The problem is, you’re constantly moving the blast gun around. Connections can loosen over time. Make sure your blast hose is making a good seal at both ends. A small leak around the end of the hose can cause a loss of suction.

We always put a quality hose clamp snugly on both ends of the blast hose on our cabinets. This is true whether we’re running a siphon tube or a metering valve. A couple hose clamps ensures a solid connection at all times. They are cheap insurance.

Don’t Put the Blast Hose Too Far Down the Siphon Tube

On many of the economical cabinets the siphon tube is a tube within a tube design. There is a small gap between the two tubes. This space between the two tubes allows air to be drawn into the siphon tube . This flow of air helps carry the abrasive to the gun. Generally you should avoid blocking this gap . Let it breathe. Don’t cover the top of the tube with media. Likewise, don’t push the siphon hose all the way down on the tube as far as it will go. Doing so blocks the air gap. This can lead to reduced abrasive flow.

We recommend placing the blast hose down about halfway to 3/4 of the way down before securing it in place with a hose clamp. Leave a gap of 1/4″ to 1/2″ for air to flow down the tube. Try not to bury the top of the tube in your media. The basic siphon tube is a terrible design; don’t do anything that could further degrade performance. Of course, the best thing you can do is eliminate the siphon tube altogether.

Other siphon tube designs feature an air hole or separate air tube design. Avoid blocking air flow to these areas for the same reasons.

8. Free That Siphon Tube

Most cheap blast cabinets with a simple straight siphon tube feature some sort of clips to mount the tube. Sometimes unmounting the siphon tube and just sticking it down vertically in the abrasive (like a drinking straw) works well. This allows you to move the tube around easily and makes it easy to quickly clear any clogs. It also helps you keep any air gap open. You’ll often be able to find a position that allows for improved flow. You do need to be a little more careful when moving around so you don’t completely pull the tube out of the blast media.

We hate to repeat ourselves, but the siphon tube design is pretty terrible. We highly recommend a gravity fed metering valve upgrade instead to get the most out of your cabinet.

9. Make Sure You Have Adequate Air Flow

Choosing a suitable air compressor for blasting is critically important. Make the wrong choice and no amount of blast cabinet upgrades or improvements will help. Air consumption rates may be the most overlooked issue for people new to abrasive blasting. Most people focus on air pressure. Air pressure is only one small portion of the equation. How many CFM, or Cubic Feet Per Minute, of airflow your compressor puts out at a given air pressure is the most important air compressor specification.

Most siphon blast guns have at least a double digit CFM air consumption rating. Guns needing 15 to 20 CFM aren’t uncommon. You may be able to blast a little with less than 10 SCFM but it is slow and not fun. With typical siphon guns, the further below 10 CFM of airflow you fall, the less likely you’ll be to see any media flow at all. If you’re attempting to blast with a tiny air compressor putting out 2.0 SCFM @ 90 PSI you’ll get little to no sustained media flow.

Remember, having adequate output from the compressor is only half the battle. Ensure your lines and fittings allow that airflow to reach the blast gun. We recommend at least 3/8″ air hose. Anything that impedes air flow between the compressor and blast gun can cause flow issues. Full or clogged separators, problems with regulators and other issues in the air line are all possible.

If you suspect an issue within your compressed air supply you can verify it with a quick check. Temporarily connect an appropriately sized air hose directly to your air compressor. Be sure to bypass all traps, regulators and other components. If you notice a significant increase in air volume compared to your normal setup, check your air supply system for obstruction, faulty components or other issues.

10. Is Your Blast Media Worn Out?


All blast media wears out over time. After all, the process is essentially slamming media against a surface at a high velocity. How long abrasive lasts depends on a wide variety of factors. Blasting at higher pressures wears out media and equipment more quickly. Spent media used beyond its useful life can sometimes flow differently and becomes too fine to work well. Some people mistake poor performance with poor flow. If your media is still flowing, but not working as efficiently, it may simply be worn out.

In Conclusion: Putting it All Together To Fix Siphon Blasting Flow Issues

Figuring out why your siphon blast cabinet has poor media flow is like diagnosing any other problem. You need to use a combination of common sense, experience and detective work.

Our approach to trouble shooting is slightly different depending on whether we’re concerned with used or new sandblasting equipment. With brand new equipment worn parts shouldn’t be an issue. Problem solved right? There’s no way it could be an issue with the gun, correct? Not so fast. If you’re experiencing issues with new equipment, you can’t rule out defective equipment. It isn’t very common, but there can be issues right from the factory.

Troubleshooting a new setup can be a bit more challenging because you don’t have the luxury of knowing what worked before. You also need to consider the possibility of improper installation and media selection. Are things properly set up. Are the blast media, nozzle and compressor properly sized in relation to one another?

Troubleshooting older equipment that worked previously can be a little easier. At least you have an idea of what was working and what might have changed. This is where common sense comes into play. Sometimes thinking what recently changed can be a huge help.

Did the flow issues start suddenly on older equipment? Are you using the same abrasive you’ve always used? If so, the odds are you’ve got a blockage or a hole finally worn through an important component. If the problems arose suddenly after switching to a new media type or size then you might need to look at your media choice, the blast nozzle size or make other adjustments.

The good news is, once you’ve gone through the tips in this post you’ll be a pro. This post should help you troubleshoot many common media flow issues in your siphon style sandblaster. We’ve created a handy one page summary of the main points of this post. Feel free to download it and print it out for a handy reference in your garage, workshop or work area.

Download our free printable troubleshooting sheet below!

Do you have a blast cabinet related question you need help with? Contact us and we’ll do our best to help!

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