Picking Grit Size For Sandblasting

How do you pick a grit size?

A quick and simple answer is to pick a maximum grain size that is about equal to the thickness of the coating being removed.  So if what you are trying to remove is 1.6 mm thick (.063 inch), you would choose a 12 grit.  A bit finer size is often used as well, because you get better coverage as the finer particles fil into grooves, cracks, voids in the surface that the larger particles will not fit into.

A more detailed answer involves selecting a grit size based on the surface profile you are attempting to achieve on your substrate.  A quick guide can be found here.

Keep in mind that the more rounded a particle is, the more contact area it has with your substrate, and this can speed up cleaning rates.  Glass bead is a good example of a cleaning abrasive, whereas the more angular and blocky aluminum oxide particle is useful for actual surface preparation.  (Where you want the abrasive to leave a surface profile for a future coating).

 

 

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Surface Preparation vs Surface Cleaning

Question:  Are You Blasting for Surface Preparation or to Clean A Surface?  The difference between the two is critical for success.  Lets quickly look at them one at a time.

Surface Preparation:  If you are blasting for surface preparation, you are trying to remove all contaminants, as well as prep your substrate to receive a coating.  This means going to the manufacturer of your coating  to find out the ideal surface profile for the coating you are going to use.   Too much profile will result in premature coating failure after the liquid in the coating dries and shrinks, exposing the peaks of the surface profile.  Too little of a profile will also result in premature coating failure, causing the coating to essentially peel off early, as the coating cannot get sufficient “hold” onto the surface.

This is why many painters will buy paint only from reputable paint companies, as they have the technical data on their coatings and can quickly tell you what your surface profile needs to be for maximum adhesion and lifespan.  The percentage of liquid to solids in any given coating will dramatically affect how much a paint shrinks when dry – this is not the time to guess!

Commonly purchased abrasives for surface preparation often include aluminum oxides, crushed bottle glass, coal and copper slags.

Cleaning Preparation:  Here the goal is to simply clean the surface without changing the substrate.  Often abrasives like glass bead, walnut shell, or plastic grit can be used in such cases, depending on your grit size, pressure, angle, etc.

 

 

 

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What Is A Grit Size – Quick & Dirty Answer

When it comes to choosing a grit size, it helps to have a mental image to guide you.  The following explanation is a “quick and dirty,” non technical, simple and easy explanation.

First, know that our ANSI grit sizes will match sandpaper grits in a hardware store.  So a 60 grit sandpaper will have particles very similar to our 60 grit loose abrasive – regardless if it is a silicon carbide or aluminum oxide.  If you prefer to view a physical representation, go to a hardware store and take a close look at the particles on a variety of sandpapers.  You will see for example, that a 600 grit sandpaper has very fine particles imbedded in it, whereas an 8 sandpaper has coarse, larger particles.

Second, imagine you held a small 1″ x 1″ square piece of mesh (screen material) in the palm of your hand.  An 8 grit will pass through a screen with 8 holes in that 1″ square mesh.  A finer 240 grit will pass through a screen with 240 smaller holes in the same 1″ square mesh.   In other words, the finer the particle size, the smaller the holes must be on that 1″ square piece of mesh for particles to pass through, and that means there will be lots more holes in the mesh.

Bottom line:  An 8 grit is much coarser than a 360 grit.  Low number = coarser grain size.  As grit size increases in grit number, it decreases in physical size, becoming finer and finer until we are dealing with polishing powders.

We sell sandblasting grits and powders online in a variety of different abrasive medias.

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CHOOSING A SANDBLASTING CABINET

Whether you’re searching a sandblasting cabinet for your shop or for your personal garage, there are several things to consider before purchasing this important industrial tool.

Sand blaster cabinets, also known as media blast cabinets or abrasive blasting cabinets, are metal boxes with openings, windows, and attached gloves which allow you to use sand or another abrasive media form, to smooth rough surfaces, remove imperfections in metal, and scrap of old paint or rust.

Yet, while all sandblast cabinets are made for the same general purposes, there are many different specifications.

Types of Sand Blasting Equipment

Sand blasting machines are made in a few different styles: standard, split-level, and portable. While all three styles operate in similar fashion and can use various types of blasting media, there are significant differences to each. Choosing which style will work best for you depends on how often you want to use your sand blaster and for what purposes.

Standard

These fixed cabinet blasters have openings in the front and on both sides of the machine. They’re great for working on small to medium sized objects.

Split-level

Split-level blaster cabinets are also fixed and have a hatch top that opens upward on a hinge to create a larger work space. These cabinets are useful when working on larger or heavier objects.

Portable

Portable sand blaster cabinets, sometimes called mini sand blasters, are perfect for working with smaller or more delicate objects because they allow your to make more detailed alterations. And, of course, benchtop sandblast cabinets are completely movable, making jobs outside of your workshop or garage possible. They offer remote control operation and are usually more inexpensive than other types of sandblaster cabinets for sale.

Once you’ve chosen the best sandblasting equipment style for your needs, the next decision you should make is whether you want to use a siphon cabinet or a direct pressure cabinet.

Types of Blasting Systems

The question here is how you want your machine deliver your abrasive media to the blasting nozzle.

Siphon Blasting Cabinets

Siphon cabinets, also known as suction cabinets, are best for light to occasion use, general cleaning and light production jobs. They use a gun to pull abrasive media into your blasting nozzle where it is then shot from the end of the nozzle. This requires the use of more air pressure than direct pressure cabinets because the delivery is a two-step process.

However, these systems are easier to use and install and are generally less expensive than direct pressure cabinets.

Direct Pressure Cabinets

These machines can expel abrasive media at a higher blasting flow rate than siphon blasting cabinets can because, unlike the siphon delivery, this system pushes the abrasive media out directly, making completing your jobs on average much faster. The direct pressure cabinets also allow you to lower and raise the pressure to customize the flow rate for each job you do.

Aside from these choices, there are other considerations too. Media blast cabinet details varies with each model and some have better features than others.

Details and Features

Here are some helpful tips and warnings to guide you through the details and specs of the blasting machines you’re considering.

Doors

If you’re buying a standard cabinet, make sure you know what material the cabinet doors are made of. Avoid cabinet blaster machines with plastic doors, which can warp and bow out easily and could lead to machine malfunction.

Air Compressors

Some Sandblast cabinets are sold with their own air compressor unit; however, not all are. If you’re thinking about purchasing a sandblast cabinet without an included air compressor, make sure to check the required PSI and CFM for your new sandblast cabinet to check if it with work with your current air compressor.

Construction

The metal used for frame and body construction also impacts the quality and durability of your sandblasting cabinet. While machines with lighter steel bodies are cheaper and work fairly well for occasional use, if you’re planning on using your blasting cabinet on a regular basis, you will need to invest in a machine with a sturdier construction, such as heavy gauge steel.

Lighting

Some cabinet blasters come with interior lighting. While this is not always necessary for daily use, if you want extra, focused lighting to help you in your detailed work, a sandblast cabinet with an interior light is ideal.

Assembly

Another, more humdrum consideration is whether the cabinet you’re purchasing is fully constructed or will require assembly. Sand blasting cabinets may require hours of assembly and the assembly could be quite complex, especially if you have no prior knowledge of how these machines are built and work.

So, once you’ve decided on the types of sandblasting cabinets and systems and thought about the specific features you want, you can start shopping the sandblasting machines for sale.

Personal Use

Before deciding on which type of sandblasting cabinet to purchase, you should ask yourself these questions to help you narrow your search and ensure you find the sand blasting machine that fits your needs:

· What do I want to use my sandblasting cabinet for?

· How often do I want to use my sandblasting cabinet?

· What is my price range?

· Where am I going to use my sandblasting cabinet?

Knowing the answers to these questions will greatly reduce the time you spend deciding between different sand blasting machines.

Conclusion

In the end, the perfect sandblasting cabinet for you is out there. You just need to think about what features, styles, and specification will work best for what you want to use your sand blast cabinet for!

 Next Step:  Buy Abrasive!

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Sandblasting Nozzle Sizing Chart

Use This Chart To Find Your Nozzle Bore Size:

  • #2 Nozzle has a 1/8″ orifice (3.2mm)
  • #3 Nozzle has a 3/16″ orifice (4.8mm)
  • #4 Nozzle has a 1/4″ orifice (6.35mm)
  • #5 Nozzle has a 5/16″ orifice (8mm)
  • #6 Nozzle has a 3/8″ orifice (9.5mm)
  • #7 Nozzle has a 7/16″ orifice (11.1mm)
  • #8 Nozzle has a 1/2″ orifice (12.7mm)
  • #10 Nozzle has a 5/8″ orifice (16mm)
  • #12 Nozzle has a 3/4″ orifice (19mm)

 

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Sandblasting Profiles: What abrasive should I use?

Anchor patterns are extremely difficult to approximate due to the many variables at play.  Use this information with Caution!  This is only an approximation.

The type of metal use in this example was hot rolled steel with tightly adhering mil scale.  The type and hardness of your substrate, how thick the scale is, the degree of cleaning you are attempting to achieve, experience and skill of the person performing the sandblasting, etc will all factor into your end result.

In this example, the pressure blasting was done using a 90-100 psi nozzle pressure.  The depth of the anchor profiles listed below are approximations only, and are on average.

1 Mil Profile:

1.5 Mil Profile:

2 Mil Profile:

2.5 Mil Profile:

3 Mil Profile

 

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What Is Friability?

What Is Friablity?  Short Answer:  How easily/quicky a media shatters on impact and breaks down.  Think of Friability as durability.  This is different from Hardness Rating.  Sandblasting medias may have the same Hardness rating, but completely different Friability Ratings.   One will break down fast, the other will remain intact and can be recycled multiple times.

Brown Fused Aluminum Oxides have a LOW friability, so it will break down slowly, and coarser sizes can be recycled multiple times.

White Fused Aluminum Oxides have a HIGH friabilty, but the SAME HARDNESS rating of Brown Fused Aluminum Oxides.  Even though it is just as hard as Brown Fused AO, it will shatter on impact and break down quickly, without doing as much damage/impact to the substrate.  This is one reason why it is often used for delicate glass etching, and why it makes a nice final polish for rock tumbling.  Its high hardness rating however, makes it appropriate for anti skid flooring applications, traction, etc.

Sinterblast, or Sintered Aluminum Oxide has a MEDIUM friability.  Think of it as the middle ground between Brown Fused and White Fused.  It can be recycled in coarse sizes, but will not have the long life (durability) of a Brown Fused Aluminum Oxide.  It is often preferred by Powdercoating companies due to its lower cost.  If you are just taking off mil scale prior to coating, this is a great media to try out.  It is a lower purity aluminum oxide, and cheaper in cost to manufacture due to the sintering process being less expensive than the fusing process.  THE MEDIUM FRIABILITY OF THIS TYPE OF ALUMINUM OXIDE IS WHY YOU BLAST AT A 60 PSI, AND NOT THE USUAL 80-90 PSI OF BROWN FUSED ALUMINUM OXIDE.  ANYTHING HIGHER AND YOU WILL NOT ACHIEVE GOOD RESULTS BECAUSE THE MEDIA WILL SHATTER UPON ITSELF IN THE AIR STREAM BEFORE GETTING TO YOUR SUBSTRATE.

When choosing an abrasive to use, make sure you take into consideration its friability, not just hardness rating.  Our product descriptions of our sandblasting abrasives include both hardness ratings and friability ratings.

 

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Number One Cause Of Premature Coatings Failure

The most common cause of premature coatings failure is improper ANCHOR PATTERN.

What is anchor pattern?  It is a term used to describe the roughness (etch, or profile) that is created on a surface when sandblasted.  The Anchor Pattern is measured in 1/1000th of an inch, which is called a MIL.  Paint coatings are also measured in MILS as well, making it a term that is easy to remember.

Coatings require specific mils of anchor pattern in order to properly adhere to the surface it is applied to.  Each coating will have different requirements.  Therefore, you cannot simply sandblast a surface clean and then apply paint to it without technical information on what mils profile that particular coating requires (among other factors).  If you simply guess at what profile you need, you risk premature coatings failure.

If you have no access at all to the coating manufacture’s technical data and need to make an educated guess, you will want to consider the following rule of thumb:

The mils of anchor pattern you create on the surface to be coated, should be 25 to 30% of the DRY film thickness of the total coatings system BUT NEVER greater than the dry film thickness of the primer coat unless additional coats are to be applied immediately.

Also key to understand is that the wet mils of a coating will be different than the dry mils of a coating.  Many coatings shrink tremendously, depending on the percentage of solids contained within them.  Make sure that the anchor pattern you create when sandblasting is much shallower than the total coatings when dry.

Other factors that impact the development of anchor pattern include the type of steel you are blasting (including hardness and chemical composition), how the steel was formed, prior use of the steel, the type of abrasive you use to create the anchor pattern (including size, shape, hardness, and velocity), as well as what type of blast nozzle you choose to use and how you use it (such as angle and distance to the work surface).

The chart below is a crude approximation of abrasive size to anchor pattern, and should thus be used only as a starting point only for both centrifugal wheel and pressure blasting.

  • 1 Mil Profile = G80 Steel Grit, 100 Mesh Garnet, S110 Steel Shot, 3060 Coal Slag
  • 1.5 Mil Profile = G50 Steel Grit, 100 Mesh Garnet, S170 Steel Shot, 3060 Coal Slag
  • 2.0 Mil Profile = G40 Steel Grit, 3060 possibly also 80 Mesh Garnet, 36 Grit Aluminum Oxide, 3060 Coal Slag, 3060 Copper Slag
  • 2.5 Mil Profile = G40 Steel Grit, 3060 Mesh Garnet, 24 Grit Aluminum Oxide, 2040 Coal Slag, 2050 Copper Slag
  • 3 – 4 Mil Profiles = G25 Steel Grit, 36 possibly also 3060 Mesh Garnet, 16 Grit Aluminum Oxide, 2050 or 1230 Copper Slag, 1240 Coal Slag

 

 

 

 

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Using Crushed Glass To Clean Precast Concrete

Crushed Bottle Glass Abrasive For Concrete

Armani Restoration Job SiteCrushed Bottle Glass Abrasive is used to clean and restore the exterior of this 27 story concrete building.  Using a proprietary wet and dry blasting system, the contractor dramatically changed the surface color of the building, as you can see in the photo.  The concrete surface of this building was not smooth – it did have a texture to it, so a medium grade was chosen.  The end result is a much brighter surface color to the concrete, without damaging the exterior.

Crushed glass sandblasting abrasive is the perfect choice for either light cleaning of precast concrete, or by using coarser grades, exposing the aggregate in the concrete.  A medium/fine grade was used for this job, as the customer did not want to damage the surface of the concrete.

Crushed bottle glass abrasive is a 100% recycled product, and is shipped from a variety of locations.  It contains no free silica, no heavy metals, is inert, and it blasts fast and clean.  Many times people will just leave it on the ground rather than clean it up (unless you are blasting off something toxic like lead paint or have local laws requiring cleanup).  This makes it great for blasting log homes and automobiles as it minimizes cleanup in areas where it can be left on the ground as a soil amendment.  Because this type of abrasive is shipped from all multiple points of origin, we will always try to find the closest location to your end job site when quoting.  NOTE:  If left on concrete it will be very slippery!

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