If you have installed a new wet blast cabinet or are trying to use it for a different application, it is possible that you may be googling your question. We often get such questions from customers and have compiled this list of Frequently Asked Questions on Wet Blast Cabinets to answer such queries and help users with correct information.

Frequently Asked Questions

Wet blasting is a process of stripping and cleaning surfaces by propelling a mix of water and abrasive media onto the blasting surface. A wet blast cabinet uses the wet blasting process inside a sealed enclosure.

Dry blasting is a process of stripping the surfaces of paint or corrosion by propelling dry media at high pressure onto the blasting surface. A dry blast cabinet uses the dry blasting process in a sealed enclosure.

Let us understand how a wet blast cabinet differs from a dry blast cabinet.

Wet blast cabinet: A wet blast cabinet is a compact enclosure where the components can be cleaned or stripped of paint, rust, oxidation, grease, or other contaminants. The used slurry (mix of water and abrasive media) is collected in the sump and can be filtered and recycled multiple numbers of times. Depending on the abrasive media used, the wet blasting provides the surface an etched anchor profile, a smooth finish, or a polished finish.

The slurry pump pumps the slurry into the blast gun and the compressed air entering from the other port propels the slurry onto the blasting surface. The blasting velocity can be varied. In wet blasting, the water molecules envelop the media particles and act as a dampener resulting in a gentle, yet efficient cleaning and resurfacing of the component.

The major advantage of wet blasting is the smooth, uniform, and feathered finish that the water provides, the cool temperature that protects delicate surfaces, and the cleaning and degreasing through the use of water.

Dry blast cabinet: A dry blast cabinet externally looks similar to a wet blast cabinet and uses compressed air to propel the abrasive media onto the blasting surface inside the compact enclosure. The main difference between the wet blast cabinet and the dry blast cabinet is that there is no water used in the dry blast cabinet. The used media is collected by a heavy-duty dust extractor and collection hopper that is attached to the dry blast cabinet. You can use a dry blast cabinet for stripping the surfaces with old paint, coating, and rust and creating the required anchored profile. However, if you need a smooth surface finish, the dry-blasted components need to be polished after taking them out of the dry-blast cabinet.

Also, see “Why should I prefer a wet blast cabinet over a dry blast cabinet?”

A dry blast cabinet may operate either with a siphon system or with a pressurized vessel. The compressed air is used for pressurizing the abrasive media and a dust collector is required to collect the dust produced during blasting. A dry blast cabinet costs less than a wet blast cabinet and you can use a wide range of blast nozzle sizes if you have a higher-capacity compressor.

A wet blast cabinet uses a mix of water and abrasive media (slurry) instead of dry abrasive media. The slurry is pumped into the blasting gun and compressed air propels it onto the blasting surface. The blasting velocity can be varied. Wet blast cabinets work dustless and hence a dust collector is not required. However, it needs a mist extractor to maintain clear visibility inside the cabinet.

So why should you prefer a wet blast cabinet over a dry blast cabinet?

The answer is a wet blast cabinet has definite advantages over a dry blast cabinet.

  • In a wet blast cabinet, the water in the slurry suppresses the dust and hence a dust collector is not required. However, a wet blast cabinet is fitted with an extractor fan to take away the mist produced during blasting.
  • A wet blast cabinet can strip or clean the surface to remove paint, rust, coatings, oxidation, grease, or other contaminants and give the surface the required finish or anchor profile in one cycle.
  • The low blasting velocity and good visibility of the blasting process in a wet blast cabinet give the operator better control over the blasting work and surface quality.
  • The gentle and smooth blasting in a wet blast cabinet preserves the geometric and dimensional accuracy of the critical components and preserves the surface accuracy.
  • The spray of the slurry that comes out from the blasting gun is even and ensures the components blasted in a wet blast cabinet have an evenly treated surface and a consistent anchor profile. There is no aggressiveness or harshness in the blasting method used by a wet blast cabinet.
  • When using a wet blast cabinet, you need not worry about the impregnation of the abrasive media particles into the component surface since the water in the slurry flushes out the abrasive media during blasting. Components used in industries like aircraft and aerospace do not want any abrasive particles embedded into their surface.
  • The presence of water in the blasting abrasive media in a wet blast cabinet eliminates the risk of frictional heat and spark due to the friction between the abrasive media and the blasting surface.
  • The wet blast cabinet works quietly at a much lower decibel and does not disturb the nearby workers.
  • The slurry used in a wet blast cabinet causes low wear and tear and the result is the life of the consumable parts like the nozzle and hose is 7 to 8 times more compared to those in a dry blast cabinet. The rubber gloves used in a wet blast cabinet have a good life.
  • The consumption of abrasive media in a wet blast cabinet is less compared to a dry blast cabinet.
  • The presence of water in the slurry makes reclaiming the used abrasive media easy.
  • A dry blast cabinet occupies more space due to the addition of the dust extractor, whereas a wet blast cabinet is more compact for handling the same sizes of components.
  • One factor to keep in mind in wet blasting is flash rusting. Steel or iron components blasted in a wet blast cabinet have a water layer on their surface and will trigger flash rust. However, the use of a rust inhibitor will provide protection against flash rusting for a period of up to 72 hours. The components processed in a dry blast cabinet can also develop flash rusting when the protective coating is removed in the blasting process and it is exposed to the moisture in the atmosphere.

Choosing the correct abrasive media for wet blasting is very important because an inappropriate media choice will not provide the desired finish. It can not only cause rework but in some cases may cause damage to the blasted surface.

The factors you have to consider when selecting an abrasive media for wet blasting are the grit size, shape, hardness, and density of the media, and blasting velocity. Factors like friability (tendency to break into smaller particles under force), recyclability, and cost are also important.

Size: The blast media comes in different grit sizes. Larger media particles will make deeper depressions but lesser depressions per unit area resulting in a rough surface finish. Smaller abrasive particles make shallow depressions but more impacts per unit area resulting in a finer surface finish.

Abrasives media is measured by the mesh size through which it was passed and the mesh can be as coarse as 20 holes per square inch or as fine as 100 holes per square inch. 40/70 media indicates that it was passed through a mesh having 40 to 70 holes per square inch. Hence a 20/40 media is coarser compared to 40/70 media, and a 40/70 media is coarser than a 60/100 media.

It is not possible to use a coarser media and complete the work faster. 20/40 media can blast more aggressively than 40/70 media. But, if you compare the number of media particles in one pound of 20/40 media and one pound of 40/70 media, one pound of 40/70 media has more media particles. Hence, when you use 20/40 media less number of particles are in action and when you use 40/70 media, more particles are in blasting action. Hence, the time taken for blasting will be almost the same, but there will be a difference in the anchor profile or finish obtained. Also, the cost of using coarser media will be more since more media is consumed.

Shape: The shape of the abrasive media particle can influence the outcome of the surface profile and the shape can be angular, sub-angular, sub-rounded, or rounded. Angular media particles like crushed glass or garnet have a rough surface with jagged faces and can strip coatings, rust, and contaminants faster. They can leave a deeper anchor profile/pattern on the surface.

Sub-angular or semi-irregular media is similar to the angular type but has less jagged faces and sharp points compared to the angular type.

Sub-rounded abrasive media is almost smooth and has only a few jagged edges and sharp points.

Rounded abrasive media has a smooth surface all over and it can even be spherical. Glass bead is a good example of rounded abrasive and is used for removing mill scales, and thin coatings, and to get a good finish or a peened surface.

Hardness: The hardness of the abrasive media is indicated as Mohs numbers (Mohs hardness scale) and the hardness number ranges from 0 to 10. The number 10 indicates the hardest and 0 indicates the softest.

Hard particles will create a deeper surface profile and softer particles will create a smooth and fine finish. A softer media is preferred for cleaning grease, oil, and grime and removing light coatings, and when an anchor profile is not required on the substrate. On the other hand, a harder media is used for removing corrosion, and rust, or for creating an anchor profile on the substrate.

Density and velocity: The density and velocity of the abrasive media also have an influence on the blasting. Denser abrasive particles blasting the surface at higher velocity will leave a deeper anchor profile compared to abrasive particles with lower density blasting at lower velocities.

An abrasive media with higher density can store more kinetic energy in it and hits the surface with high impact to form a deeper surface profile, whereas an abrasive media with low density can store less kinetic energy, and hits the surface with low impact to form a lesser deep profile.

Using the same abrasive media, the surface finish can be slightly modified by varying the blast pressure, but this will not affect the basic characteristic of the media.

Let us summarize the characteristics of the abrasive media in a table

No Blasting media Mohs hardness Density/specific gravity Recyclability
01 Garnet 7.5 to 8.5 3.5 to 4.3 Up to 5 times.
02 Crushed glass 5 to 6 2.5 Depends on the grit size.
03 Glass beads 5 to 6 2.5 Up to 30 times.
04 Aluminum oxide 9.0 3.94 to 3.96 Up to 50 times
05 Soda bi-carbonate 2.5 2.2 Single-use

Different types of anchor patterns and surface profiles- When the blast media impacts the surface, it creates a profile in the shape of the media on the substrate. A media in a spherical shape create a surface with mini dimples (like the surface of a golf ball) and this finish is called peening.

Angular and sub-angular abrasive media can create deeper and well-defined anchor profiles on the substrate. If you observe an anchored profile under a lens, you can find a surface full of valleys and peaks. The indentations created by the media are the valleys and the surface that is pushed up due to the indentations is the peaks. Each type of coating requires a particular anchor profile or surface roughness for best results. You can say that during powder coating the coating substance fills the valleys and covers the peaks of the anchor profile. If the peaks are too high, they will start showing up and can catch rust. And, if the valleys are not sufficiently deep, then they cannot hold the coating (the coating cannot adhere properly).

Recycling and cost of abrasive media- Some abrasive media have a high initial cost and if it can be recycled more times, then its ability to recycle will offset its high initial cost.

The advantage of using a wet blast cabinet is the used abrasive media can be reclaimed and recycled easily and without the need for a dust collector.

Soda bi-carbonate can be used only once, garnet can be recycled up to 5 times, glass beads can be recycled up to 30 times, and aluminum oxide can be recycled up to 50 times. Crushed glass does not have good recyclability and many times it is used as a single-use media.

How many times a media can be recycled depends on its Mohs hardness number, the grit size of the media, and the surfaces blasted with that media. If the surface blasted by the media is several times harder than the media, this will reduce the number of times it can be recycled. The blast velocity also has a small influence on the number of times a media can be recycled.

Each abrasive media can withstand a maximum blast velocity and when this maximum value is breached, the abrasive media can lose its capacity to recycle.

The shape of the blast media also has an influence on the recyclability of the media. Spherical media like glass beads used within its recommended blast velocity can absorb the impact better since it spreads more evenly over a spherical surface. So, glass beads have good recyclability.

The different finishes you can achieve in a wet blast cabinet are:

  • Surface with a deep anchored profile for adherence to powder coating or painting
  • Very smooth surface finish for articles that have an aesthetic value.
  • Surfaces with a peened appearance (like a golf ball) give a crisp look to the component
  • Smooth surface finish as preparation for hard chrome plating, galvanizing, tin plating, etc.

The surfaces that can be blasted in a wet blast cabinet are:

  • Metal surfaces like aluminum, steel, stainless steel, and other metals with rust, old powder-coating, paint, and contaminants
  • Wooden surfaces with stain, coating, and contaminants
  • Softer metals, plastic, fiberglass, and PVC with dust and contaminants on their surface

The following can be cleaned in a wet blast cabinet

  • Metal surfaces with rust, old powder-coating, paint, and contaminants
  • Metal and non-metal surfaces contaminated with grease, and grime
  • Surfaces of a plastic injection mold
  • Surfaces with burrs caused during machining on CNC machines or machining centers.
  • Dirt from the holes of printed circuit boards (PCBs)

Adding media into the wet blast cabinet is very simple, and normally, you will be able to pour around 50 pounds of media into the hoper of the blast cabinet. But, it is better to refer to the product manual to know the quantity of media you can add to the wet blast cabinet.

To add the media to the wet blast cabinet, you have to open the grate and pour the media into the hopper. Then, fill the hopper with water up to the specified level and close the grate.

Soda bi-carbonate that is specifically manufactured for sandblasting can be used in a wet blast cabinet. It is granular in shape (unlike soda bi-carbonate in powder form) and has a moisture protection coating that stops it from getting dissolved in water. Soda bi-carbonate can be used as a blasting media for degreasing and cleaning parts, resurfacing delicate surfaces, medical and aerospace components or antique objects or sculptures, where a smooth surface finish is required. Soda bi-carbonate is a single-pass media and cannot be recycled.

The wet blast cabinet you are planning to buy should have the CFM (cubic feet per minute) of air at a PSI (pounds per square inch) recommended by the wet blast cabinet manufacturer.

The three important things you should look for in an air compressor are the pressure in PSI, the flow of air in CFM, and the HP (horsepower). The air compressor normally comes fitted with an air storage tank. You must also enquire about the duty cycle of the compressor.

What is duty cycle: The low-end air compressors are of the reciprocating type and they have a duty cycle. Suppose you consider a compressor run time of 10 minutes, a 50% duty cycle implies that every 5 minutes of the compressor running should be followed by a rest of 5 minutes. This duty cycle does not affect small-level operations since you will be drawing the air from the air tank and as soon as the pressure in the air tank comes below the set level the compressor starts and fills the tank. Also, the wet blasting process in the cabinet is interrupted for the loading and unloading of the component.

Go for an air compressor with a good duty cycle and it may be wise to purchase a compressor of a step higher size (50% higher than the recommended size for the wet blast cabinet) to cover pressure losses and future requirements.

However, if you are operating several higher size wet blast cabinets simultaneously, you can either plan for a multi-stage/multi-cylinder reciprocating compressor with a higher duty cycle or a rotary screw type compressor. Rotary screw type compressor can run continuously and is recommended for industrial use, but its cost is appreciably more.

Flash rusting is the rust that forms on the wet-blasted metal components within a short time of undergoing wet blasting. The reason for flash rusting and its speed of happening depend on the salt contamination of the surface or high humidity or a combination of both.

The flash rusting can be prevented in components processed in a wet blast cabinet by following the three simple steps.

Ensure the water from a clean source is used: If you are sourcing the water from a well there are chances that the water may contain high chloride. And like oxygen in the air, high chloride can react with the metal surface to cause rust. Using water from a clean source, like the treated water supplied by your town water can minimize flash rust.

Ensure the substrate is free from contaminants: The substrate that you are processing in the wet blast cabinet may already have some deep particles of rust on it and as soon as the wet blasting is completed the rust will restart at these locations. The blasting process used in the wet blast cabinet should be able to remove such particles of rust from the substrate.

Take the help of a good rust inhibitor: There are branded rust inhibitors that can be mixed in the water used for wet blast cabinets. These rust inhibitors can create a barrier on the surface and prevent the oxygen in the atmospheric air to start rusting the surfaces.

The amount of rust inhibitor consumed by a wet blast cabinet depends on its water consumption. A wet blast cabinet that uses less water will consume less amount of rust inhibitor and that is saving money for you.

Blow off the moisture with clean compressed air: Clean compressed air can be used to blow off the moisture as soon as you take out the component from a wet blast cabinet to minimize flash rusting.

The media which is completely recycled and used is called ‘spent’ media and the spent media needs to be disposed of. The media used in wet blasting may not be toxic by itself but during wet blasting, it can get mixed up with the dust or contaminants stripped during blasting, and depending on the contaminant removed, the media may become toxic. Hence, every care is required for the disposal of spent media.

The method of disposal of spent media depends on the place of your work and the rules framed by the local governance. It is best to submit a sample of the spent media to the local waste management center and take their advice for proper disposal. Doing this will keep you on the right side of the law and save you from unnecessary legal issues.

Problems faced in wet blasting due to the source of water are not very common, but they cannot be completely ruled out.

The water you use for the wet blast cabinet may have been sourced from the city water tap or a well or spring, but the chemicals and minerals contained in them can have an influence on the outcome of the wet blasting.

The PH (Potential of Hydrogen) is a measure of how acidic or basic the water is and the scale ranges from 0 to 14. Normally, the water we source from a tap or a natural spring has a PH value of 7 and a PH value of 7 is considered neutral.

The PH is a measure of the relative amount of free hydrogen and hydroxyl ions in the water. The acidic water contains more free hydrogen ions, and the basic water contains more free hydroxyl ions. If the PH value drops below 7, water is termed acidic (it has more hydrogen ions compared to neutral water) and if the PH value rises above 7 the water is termed basic (it contains fewer hydrogen ions compared to neutral water).

When you wet blast aluminum alloy, steel, or stainless components in the wet blast cabinet and the finished component appears discolored (slight brownish or purplish shade), it may indicate the PH problem in the water.

Test your water for PH value, and if the PH is above or below 7, discuss with your local chemist to solve the problem.

If the PH of the water is found correct (around 7) then the culprit can be the media. In rare cases, a particular media used by you may react with the water and modify its PH value.