Category Archives: Glass Beads

News and technical data on the use of sandblasting with glass beads.

Picking Grit Size For Sandblasting

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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

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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

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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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Sandblasting Profiles: What abrasive should I use?

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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?

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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

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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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Cleaning Bronze with Glass Bead Blasting

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Here is a great video and article on glass bead sandblasting.

This company is using a fine glass bead to remove bronze patina from outdoor monuments, which restores them to their original state.  I would have liked to see some before and after photos of the monuments, but the article does contain a video clip where you can see (from a distance) the cleaned and uncleaned areas.

Workers from Sterling-based Mercer Lettering and Monument Works have been  sandblasting and pressure-washing the granite stonework over the past few weeks.  The granite stones also have been treated with an organic acid designed to  preserve the grass below while removing fungi from cracks in the stone.

On Thursday, they aimed nozzles at the bronze plaques and sprayed them with  fine glass particles. That removes the patina, or green and brown film, that has  developed on the metalwork over time without damaging the metal below it, said  Graeme Everson,  the company’s owner and a National Guard veteran.

“It brings back the shine of the bronze as it was when they put it up,” he  said.

The article does not discuss the size of the glass bead used, nor the exact machine they used, but the video shows the actual operator blasting.  You can therefore see the blast pot and the operator’s protective head gear which is useful for those just starting out and not familiar with protective gear.

To see the full original article by By ALISON SHEA of The Bulletin: Jewett City monuments gleam like new – Norwich, CT – The Bulletin http://www.norwichbulletin.com/news/x1474279320/Jewett-City-monuments-gleam-like-new#ixzz21poiW0in

© Copyright (c) ALISON SHEA The  Bulletin Originally Posted May 18, 2012

 

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