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Paint removal and corrosion treatment of cars, motorbikes and aeroplanes.

Professional and amateur restorers from all over the UK bring us car bodies, panels and parts for removing paint and/or corrosion. Since we started the blasting side of our business in 1985 some interesting items have passed through our works. D-Type and XKSS Jaguars, Ferraris, Maseratis, Aston Martins and Lamborghinis are some of the more exotic classic cars we've helped to restore, but we also cater to more modest vehicles, MGB, Morris, Rover, Triumph, the list is endless. From Formula 1 to hot rods, from vintage to classic, from army quad bikes to bubble cars, from tractors to Aston Martins.  We treat cars made of steel, aluminium and even glass fibre and we apply the same methods when treating motorbikes and aeroplanes. 

So...............What is media blasting and bead blasting?

Like most industries there are generic terms and more specific technical terms to describe various processes.  So lets try to clarify what the terms mean.

There are, of course, many variables and special applications. This is not intended to be all encompassing but merely a layman’s guide to Blasting terminology.

You have heard the term Sand Blasting, which was invented over 100 years ago but really took off around 50 years ago, blasting with sand.  Sand does a great job and it was used for years, even though they knew as long ago as the late 40’s that sand had hazardous contents in the form of free silica. It is the tiny particles that are released when the sand breaks up when it hits something hard that are dangerous to your respiratory system. So Silica bearing sand was banned from use in any pressure vessel for blasting around 1980 and some manufacturers put big labels on their equipment advising it was illegal to use sand. Unfortunately that does not stop the ill informed or unscrupulous from still using sand and it is cheap, but at what cost to your, or their, health?

So, people commonly refer to Sand Blasting but it should really be referred to as Abrasive Blasting which covers a wide range of legal alternative materials available today.

Media Blasting is commonly used as a generic term for almost any blasting product, so is a bit vague, whereas Plastic Media Blasting is what it says, angular pieces of particular types of plastic that can be used for certain blasting applications.

Shot Blasting is also a generic term used by many people to describe blasting but can encompass almost any type of blasting. Shot Blasting is generally the term applied to the use of steel shot, tiny steel balls like shotgun pellets, that come in a whole range of sizes and are used for various purposes, too lengthy to deal with here.

Shot Blasting can also be referred to when really meaning Steel Grit Blasting, which instead of tiny steel balls it uses angular pieces of steel similar to many of the other abrasive product with angular particles that are sharp and aggressive.  This is generally used for cleaning industrial work and heavy steel components.  Steel grit comes in a range of sizes but tends to be on the larger side for most industrial applications and just what you do not want on you classic body shell.  We have seen some shells that have been blasted in this way, it is too agressive and they were ruined with distorted panels everywhere.

Bead Blasting is one we hear all the time, used again as a generic term to describe almost any blasting process. Bead Blasting is usually associated with Glass Bead Blasting, tiny glass balls  in a whole range of sizes and used generally for cleaning and finishing aluminium castings, for example, engine casings, plus stainless steel items, and can also be used to finish steel items if there is a special requirement.

Now lets look at the processes in more detail, and how they work.

Glass Bead Blasting

Bead blasting is generally the term used to describe the cleaning process whereby spheres of glass are propelledat the surface need to be cleaned in a fast moving stream of air. The glass bead are usually round and because they have no sharp edges they clean by virtue of the kinetic energy resulting from their mass and velocity. It is like millions of tiny hammer blows. The glass bead is comparitivley hard to other blast media, and it needs to be because it is effectively hammering the surface of the item being cleaned. This hammering or 'peening' action moves the surface of the metal around, at a microscopic level, and fills the tiny fissures in the surface of the metal that you can see through a microscope (they exist even on highly polished surfaces). By filling in these microscopic voids you reduce the ability of foreign corrosive elements and dirt from getting into the surface of the metal. Not only will this help reduce deterioration of the surface over time but it is also used to create a surface that is more hygenic in certain applications, such as the food preparation industry and medical equipment. 

The surface you want to finish with bead blasting will almost certainly benefit from cleaning by other methods first, such as a fine abbrasive blasting, to remove as many contaminants as possible, but it will depend on the item and the finish required. If you do not pre-cleanan already corroded item like an alloy wheel, then the peening action of the glass bead will trap the corrosive meterial in the surface of the aluminium leading to early deteriation of the metal because of the contaminant fighting to get out.

Glass bead blasting normallty leave a satin finish on the surface you have treated, if you use glass bead mixed with water and air in a pressurized system it is called vapour blasting and give more of a sheen finish, particularly on alloy parts. However, this is not the same as a polished finish which is normally achieved by a buffing process with the use of polishing compounds.

Beware, the 'peening' that the metals under go during the glass bead blasting process is not always applicable on certain items such as thin sheet metal. Due the hammering effect described above and remoulding which occurs, this can result in warping and distortion, that can be irreversable . 

Plastic Media dry stripping

Plastic media dry stripping is exactly what it says; it uses millions of tiny particles of angular graded plastic propelled at the surface to be paint stripped in a stream of fast moving compressed air. The paint is removed by the chipping and scratching action of the sharp edged pieces of plastic.

The sharp angular particles of plastic are relatively soft and light-weight. It is propelled at much lower pressure than most other blasting processes because the plastic deforms and breaks down very quickly if it hits the surface too hard. As the plastic media is some 20+ times more expensive than normal blasting abrasive there are very good commercial reasons for us to use the plastic media at low pressure and recover it for re-use, but its unique properties make it ideal for this type of cleaning.

On steel panels the plastic does not mark the surface of the metal and so allows the removal of the paint, filler, glue and some of the old types of underseal if they are dry and friable, from thin steel panels (and some other base materials too). There is no distortion of the steel because there is no heat generated, the plastic media is not hard enough to break the surface of the steel.

You can then see everything uncovered, even how the panel was last prepared, all the grinding marks and abrasive rubbing down marks can be seen, even phosphate dipping used during manufacturing can be seen on the car bodies as a tide line after plastic media paint stripping. Where lead loading has been used on the panel joints by the manufacturer, the plastic media is hard enough to disturb the surface and create a slightly  roughened profile in the surface of the lead, but it does not remove the lead.  A quick sand over with a dual action sander reduces the profile in seconds.

Plastic media will not remove body seam sealer or the modern underseals economically - they are too resilient and designed to resist abrasion. Apart from damaged or poorly adhered areas, which it will identify, why would you want to remove the factory seam sealer? (The exception being cars for racing which have to be seam welded for strength). Some of the modern paints have plasticising additives, which make them very tough to remove, but there is no safer way of removing paint from panels.

Finer Plastic Media and its uses

Plastic media can be used on aluminium panels and fibreglass too. More operator skill is required and smaller grades of plastic media and lower pressure are required as these surfaces can be damaged because they are softer than steel. Aluminium panels will exhibit a profile on the surface because the plastic media is hard enough to disturb the surface of the aluminium, but only in microscopic terms. To minimise the risk of distortion and profiling a fine grade of plastic media with small particle sizes is used.

Thermal Image of plastic media blasting

 

The first picture above, is a thermal imaging camera recording plastic media blasting, while the second below is of a grit blasting process. What it shows is the amount of heat generated by the blasting processes.  The brighter and more extensive the orange/white area, the greater the heat generated by the friction created by the cleaning process on the metal.                  

Thermal Image of grit blasting.

Corrosion Removal

Once all the paint, filler and underseal, glue, sound deadening, oil, grease and all the other problem materials have been removed using the plastic media blasting process, the corrosion should be visible and can be dealt with.

To remove the surface corrosion an abrasive that is hard enough to scour the surface clean must be used. It needs to be sharp and angular to cut into the corrosion on the surface of the metal but fine enough to prevent the hammering or peening action that has the potential to damage the panels. This is where a skilled operator is essential to minimise the risk of distortion.

Because most of the work has already been done using the plastic media, the risk of overworking the surface with the hard abrasive is greatly reduced. There is less need to dwell in one place with the abrasive blast stream, it is this dwell that generates a tiny amount of heat and the stressing action that can cause distortion to panels. The finer the abrasive the less kinetic energy there is in the particle, to dissipate when it hits the surface. This makes removal of the surface corrosion slower but leaves a very fine profile on the surface of the steel panel, which is a good key for the paint and any filler required.

There are blasting companies around who blast car bodies and panels with hard abrasives and have to chase the 'live edge' to remove the paint and filler etc. and this is when there is considerable risk of damage. Anything rubbery in nature requires prolonged blasting and the risk of peening and heating the surface is very high, the damage irreversible. There are operators who can do it this way but they are few and far between.

Protection after corrosion removal - phosphate coating

When all the paint etc. and corrosion has been removed the item is cleaned with compressed air to remove the residues of the cleaning media. It takes large volumes of air to blow out the box sections and it is sometimes difficult to do so. The steel surface is grey in colour and  'technically clean'.   In layman’s terms that is about as clean as you can get. However, steel panels in this condition are very susceptible to surface corrosion from moisture in the atmosphere and even the sweat on your hands. It is essential the surface is protected as soon as possible but we do not think priming the surface with paint is the best solution. Primers are not waterproof in most cases for one thing and any paint should be removed locally if welding repairs are to be carried out. In addition, the primer hides the surface so you cannot see the metal to see if it is clean and sound.

We spray the body / panels / parts with a phosphate coating . It is a clear product but leaves a slight green tinge in some areas as it works into the metal. You can still see the metal surfaces - and see if we did our job properly because nothing is hidden. A coat of primer can hide a multitude of problems, and poor worlmanship. It is an advanced coating which etches into the metal drawing zinc into the surface. It also has corrosion conversion properties which help to stabilise the invisible corrosive salts that are ever present in the metal where corrosion was previously visible even though the surface is technically clean. You can weld through the treatment without removing it because it isn’t a paint. The phosphate is a chemical preparation that passivates the surface of the metal, killing corrosive salts and providing a chemical key on to which the paint molecules can lock.  This, combined with the profile (roughness) of the surface created by the final very fine abrasive cleaning process used, provide a mechanical key that further aids paint adhesion.

The phosphate treatment can be left on the surface and over-coated by any kind of paint. (In the 20 years we have been importing the product no one has reported a paint compatibility problem.) Prior to painting, the surface should be rubbed over with a Scotch-bright pad just to remove any dust or loose particles, blown off and/or solvent wiped prior to painting as you would normally do for good paint-shop procedure.

We try to handle panels with gloves after phosphating because your sweat has lots of acidic content and when you put a sweaty hand on a panel it will leave a perfect hand print in about 3 weeks, even after phosphating, and much quicker on an unprotected panel.

The phosphate coating is not intended to be exposed to direct contact with rain etc. but will protect the metal quite well for months in a reasonable workshop. A customer recently reported that a car body we cleaned, which had to be shelved for 4 years, was rolled out again and found to be in remarkably good condition, protected only by our phosphate treatment. Another brought some doors back after 12 years storage, protected only by the phosphate coating. They were not perfect, but still in remarkably good condition and all that was needed was a light blast off and re-phosphating.

If the shell does get wet on the trip home, it will look blotchy with white marks on the panels.  Put it in the paint oven and dry it off as soon as possible.   If you don’t have an oven or a heated garage, use any form of forced heat (electric is better than gas or oil because they both produce a lot of water vapour that you don’t want.). Heat up the garage, make a tent over the car to enclose the heat if need be, and put your heater in or under the shell; even an arc lamp produces a lot of useful heat. (If the wife is out, open the French doors and put it in the living room with the central heating turned up, but there is likely to be more heat than you want when she comes home and catches you!)  When it dries off it should not have deteriorated too much, but it doesn’t look nice.

So there it is.  Over the last 20 years we have cleaned hundreds of cars – from bubble cars and Morris Minors to Maseratis and Ferraris and everything else between – for owners and restorers, from the one man band to the Aston Martin Heritage Centre.  And they keep coming back, so we must be doing something right!    

If you have a car restoration in mind and have a specific query you might find the answer in our Frequently Asked Questions section.  Or call us to discuss your project 01403 752347.