Vapour and Dry Media Blasting

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All shiny and new again : credit: © Alisdair Cusick
Blasting processes are an efficient and effective way of restoring parts. Alisdair Cusick compares vapour and media blasting

Need to know

Time: 2 hours
Difficulty: 5 out of 5
Models: All Land Rovers.
Tools needed: Blasting equipment, compressor, safety PPE.
Costs: Minimum charge of £30, then price per item: four steel wheels, £120; rocker cover, £30; radius arms, £50 per pair. Heavily corroded items would slightly increase the costs.
Work safely:
• Use the right tool for the right job.
• Always be fire-safe in the workshop.
• Ear protection is required during blasting.
• Never carry out blasting outside of the cabinet and ensure the cabinet is closed and sealed before working.
• Only work through the fixed protective gloves and check they are intact and fully sealed to the cabinet.
• Never use sand as a blast media (see box).
• Get an expert to do the job if you’re in any doubt.
Thanks to: With thanks to Michael Baughan of MVB Blasting: 07833 060693, [email protected].

 

Maintenance or restoration of Land Rovers involves cleaning components back to bare material, perhaps to remove oils, paint, corrosion, or simply to cosmetically refresh a part or to get it back to bare metal to make a proper repair. There are several ways to go about that, but principally it comes down to a choice of either chemical cleaning or physically abrading the surface. Each of those alternatives brings further choices, processes and outcomes.

Chemical cleaning is a valid option, depending on the contamination. For heavy contamination or corrosion we have to remove that damaged surface physically, and we can use fixed abrasive media, like flap discs or grinding discs, or we can use loose abrasive media in blasting processes.

Michael Baughan, of MVB Blasting, explains how it works: “Blasting [blast cleaning, to be accurate] involves a loose abrasive media being mixed with water and/or air under pressure. That stream of compressed air, water and media is then fired at the surface of the part to be cleaned.” There is a seemingly endless list of blast media available, from ice, walnut shell, glass, steel, aluminium oxide, sand and even soda – but never sand (see page 102). Michael continues: “Each blasting method has its own process, abrasive characteristics and finish quality, meaning I can balance the material being treated, the level of abrasion and the desired surface finish.” The benefits are speed, thorough preparation, surface uniformity and reduced contamination, compared to mechanical abrasive or chemical cleaning. It also means no mess around the car from grinding dust, or unsociable noise from running angle grinders all weekend.

Vapour and media blasting are the processes to deal with heavier cleaning requirements. They’re more abrasive than ice blasting and each uses different methods to deliver blast media. “Vapour blasting uses water to carry the blast media, and media [dry] blasting uses compressed air, but they differ in how they work, and the materials best suited to each process,” Michael says.

MVB Blasting’s Michael demonstrates both processes, and explains how they work, which is best, and for what outcome.

 

Vapour blasting – what’s it used for?

Looking rough: Our sample test items: corroded steel Rostyle wheel, crusty Defender A-frame arm, plastic coolant reservoir, Series I aluminium rocker cover, Td5 sump. All can be refreshed.

A glimpse inside: This is the vapour blasting cabinet, opened up. The working  area and tools are accessed through gloves, looking through the inspection window to operate the blast hose.

Step one: First, the work piece is run through the parts washer to remove grease and loose surface contaminants. Degreasing components at this stage helps preserve the blast media.

Ready to go: This is the typical starting point. The part has been chemically cleaned, but ingrained dirt remains, along with decades of old stubborn paint. Blasting will restore this to as-new.

Blasting stage: Michael begins blasting the old rocker cover. He works the blast hose in circles with the hose angled at 45 degrees to lift contaminants from the surface as well as abrading it.

Big reveal: After checking for anything missed and more cleaning sessions, Michael proudly shows the end result. An original 70-year old Series I rocker cover, restored to a gleaming as-new finish.

Great on stains: This cruddy Td5 sump is heavily stained, but will be equally transformed. Chemicals may clean it, but they’d need to be relatively aggressive. Instead, Michael sets to work.

White dust: Showing quite heavy corrosion inside, there’s plenty of white, powdery bubbling. This needs to be physically removed by abrasion. Even the most aggressive chemicals are going to struggle.

Fine details: These threads are also getting decidedly crusty. Access is limited for wire brushing, but vapour blasting is perfect to sharpen up all these fine details, bringing them back to as-new.

Ideal process for it: A brief session on the threads shows the transformation. All the rough material around the threads is removed, revealing sharp, clean threads once more, along with a clean mating face.

Quick progress: Very quickly, impressive progress can be seen. It is inevitable that there will be places that are missed on the first pass, which is why regular inspection is part of the blasting process.

More blasting: On complex shapes the work piece is rotated regularly to help access and continual inspection. To avoid damaging parts, the cabinet base is made of plastic.

What dry corrosion? Likewise, corrosion is removed. The polishing effect of vapour blasting smooths the surface, leaving a pleasing cosmetic finish. Note that it only cleans, it doesn’t replace any material.

Other materials, too: The process also suits other materials, from magnesium to plastic. This Td5 coolant reservoir shows the usual staining and grime, but vapour blasting will clean this surface easily.

50/50: Michael does a half and half to show the difference. The grime is gone and the part is spotless. The polishing effect of vapour blasting avoids surface pitting.

Not for ferrous parts: To demonstrate why the process isn’t suitable for ferrous metals, Michael blasts this cast iron manifold. It cleans well, but the water causes a problem: flash-rust.

Flash-rust: Just two minutes later, the cleaned area has begun to rust before our eyes. For this reason, ferrous metals require grit blasting, not vapour blasting.

 

Media blasting – what’s it used for?

Prep first: Media blasting begins with initial cleaning, but rather than oil removal, surface corrosion is the problem. Flaking rust is brushed off first, before moving to the parts washer.

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Dry environment: The blast cabinet is set up in a similar fashion to the vapour unit, but uses air rather than water to clean components. The parts sit in the cabinet and a blast hose directs the media stream.

At work: Michael again operates the blast cabinet via fixed heavy gloves, observing through the cabinet’s inspection window. Hearing protection is essential during vapour and grit blasting processes.

Blast hose: Operating at 60psi, the blast hose can be fitted with varying nozzle sizes to control the velocity of the media. Michael is using a 6mm nozzle here, and the media is iron silicate.

Result: Starting at the wheel’s centre hub, the process removes paint and corrosion back to bare metal. Details such as the stamped lettering are revealed and the surface is slightly rough.

There are limits: Fully cleaned, we can see there is a limit to what can be achieved by dry media blasting. The inside of these wheel rims are deeply corroded. Blasting won’t repair this, it only cleans.

Ideal part: This A-frame arm is ideal for media blasting. Using a flap disc would create lots of dust, but grit blasting contains that mess, whilst cleaning this large component more thoroughly.

Process differences: steel: Two minutes blasting with two processes. Vapour (top) and media (bottom). Note the more rapid progress with media, not to mention the flash-rust issue of vapour blasting steel parts.

Process for aluminium: Aluminium parts cleaned with vapour (left) and media (right). Note the rougher surface left by media blasting on the softer material – it’s an ideal key for painting the parts.

Finished results: The group of parts after cleaning with media (left) and vapour (right). Differing processes and materials, but with the same outcome: original parts, refreshed to be used again.

 

BLASTING PROCESS CHOICES


Abrasive media

From left: Aluminium oxide, glass beads and iron silicate

The choice of abrasive media (above) used in blasting can control the level of abrasion. The media is recycled through the machine, so it wears down over time, slowly losing its grit. So, what starts as a more aggressive grit will become less abrasive, essentially wearing out. Some materials stay sharper for longer than others, but not all media needs to be highly abrasive. The desired final surface finish needs to be considered as well.

Michael of MVB has a warning: “We probably think of blasting as sand blasting, but that is the one media that isn’t used, nor should it be, for health reasons.” Silica sand, if used for blasting breaks up into tiny particles, smaller than five microns, meaning it simply floats in the air for days, rather than settling to the floor. Inhaling this dust can cause silicosis, a fatal respiratory disease that hardens the lining of the lungs. All blast media should contain no more than 1 per cent free silica.

 

Dry ice blasting (covered in more detail in a previous feature)

Dry ice blasting is best for a cosmetic refresh, removing surface contaminants without leaving a mark on the surface underneath. It’s perfect for refreshing engine bays, wheelarches and wiring on an original condition vehicle by simply removing ingrained dirt. Its downfall is that it isn’t aggressive, so whilst it will clean, it won’t touch corrosion, nor even remove any paint beyond what is already loose.

• Cleaning medium: CO2 pellets at minus 78.5ºC.
• Best suited for: cleaning original condition cars or components.
• Least suited to: removing corrosion.

 

Vapour blasting

Vapour blasting uses a blast hose and a wash hose connected to water and media reservoirs, and a compressed air supply. The operator manipulates the blast hose, wash hose or work piece via heavy rubber gloves mounted to the cabinet while looking though an inspection window.

The process uses water and compressed air at 45psi to carry the blast media/water mix through the nozzle and fire it at the work piece. Because the media is suspended in water, it flows over the work piece giving a slight polishing effect. The residual slurry collects in a sump at the bottom of the cabinet from where it is pumped back through the hose again. For inspection, a water hose in the cabinet is used to wash off the work piece. Typically the process is used on non-ferrous metals; aluminium, brass, alloys, but also plastics and glassfibre.

• Cleaning mediums: aluminium oxide, glass, from angular to rounded profiles.
• Best suited for: cleaning non-ferrous metals; alloy, aluminium, glassfibre, plastic. Slight polishing effect.
• Least suited to: ferrous metals, removing heavy corrosion.

 

Media blasting

Similar to vapour blasting, it uses a cabinet-mounted blast hose fed by compressed air. Dry blast media is fired under pressure at the work piece. As before, used media falls to the bottom of the cabinet and is recycled back through the blast hose. The abrasive media hitting the work piece shocks the contaminant free from the substrate. Being a dry process, dust is created, which is contained within the cabinet.

The blast media hits the surface and bounces off, so it can create surface pitting if used too aggressively, or on soft materials. Blast media is best used on ferrous metals to remove heavier contamination, paint, or corrosion.

• Cleaning mediums: aluminium oxide, iron silicate.
• Best suited for: ferrous metals, removing paint and corrosion, leaving a good key for subsequent painting.
• Least suited to: glassfibre, carburettors, and anywhere where a fine polished finish is needed.

 

Ferrous or non-ferrous?

To check what material a part is, and thus which process would suit, use a magnet. If it sticks, it’s ferrous, so would suit media blasting. If it doesn’t stick, then it’s an alloy, so vapour blasting is the usual choice.

 


 

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