Thursday, 27 August 2015

Basics of abrasive polishing

Always start with the least abrasive combination, i.e. a machine polish and the least aggressive foam / wool pad.
Automotive paint surfaces comprise numerous microscopic peaks and valleys, much like the profile of a mountain range. These irregularities are known as capillary structures; there may be millions of these defects per square inch. When these scratches are removed from a surface it becomes a smooth level surface that reflects light.
Polishing could be described ‘ as going through the grits’ i.e. each progressive piece of grit finishing paper or pad / polish combination you are replacing scratches with finer and finer scratches until they cannot be seen with the naked eye
Are identified by a grit number; "grit" is a reference to the number of abrasive particles per inch of finishing paper that are required to remove the scratches left by the grit scratches i.e. 2000 grit scratches are removed by using an abrasive rated at 3000 grit. You are actually replacing scratches with even finer scratches until they are invisible to the naked eye.
The ability for an abrasive to "cut" depends on the shape of its crystals or particles, not necessarily its size. A medium size, spiked abrasive will tumble and dig. However, a large round crystal won't leave a deep scratch.
A large hard abrasive may also be brittle. It will cut once and lose its edge, while a softer small abrasive will hold its edge and keep on cutting. Many smaller abrasives have wedge shaped edges protruding from triangular crystals. These can easily slice through an oxidized layer of metal.
Polishing –cutting using a compound (an abrasive process) or polishing, (a burnishing process), since the polyurethane is a thermoplastic, generation of excessive heat may cause the molecular structure to soften or expand driving the defect deeper into the substrate
Polish to a ‘haze’ - when a polish "flashes" from a liquid paste to a light semi-dry haze; its colour changes from the polish colour to almost transparent (like a thin film of Vaseline) ; the polish has then broken down and is ready for removal. The point at which a polish is fully broken down comes with experience but a good yard stick is when the polish has become clear and can easily be wiped off.
Breaking-down the abrasives – it is important to know when a diminishing abrasive polish has broken down because if you take it too far it will dust and you will re-introduce surface marring, conversely if you don’t work diminishing abrasives sufficiently they will cause surface marring; this is due to the size of the abrasive and its cutting ability, were as once an abrasive has broken down it will burnish the surface as opposed to cutting it

                                               Note: 'Product Designation' numbers refer to grit
Grit Numbers
Sandpaper or finishing paper is the most common item from a larger group of products known as "coated abrasives" i.e. Aluminium oxide. 

When talking about "grit" is a reference to the number of abrasive particles per inch of finishing paper (sandpaper). The lower the grit the more abrasive and conversely, the higher the grit number the lesser (smoother) the finishing paper
When talking about abrasive finishing paper, "grit" is a reference to the number of abrasive particles per inch of paper. It eliminates the risk of deep sanding scratches by providing a uniform grit size. This makes sense if you imagine how small the grit particles on a 1000-grit finishing paper would need to be to fit into a 1- inch square. Grit finishing paper is referred to by the size of its abrasives (i.e. 1500-grit paper) the grit you use depends on what kind of scratch you are trying to remove

Compound: 1000-1200 grit

1500 – 2000 grit

Finishing polish:
2000 – 400 grit

Materials used - most good compounds are a combination of both silica and aluminium oxide. The abrading ability of these compounds can be changed by their application method (i.e. machine speed and/or pressure used, using wet or dry and/or type of foam (different foam compositions have a differing abrading ability) Allow sufficient time for the polish to work, with a more aggressive polish a longer time period is required (approx. 3 – 6 minutes)
The speed at which the foam pad travels across a paint surface is also important, moving too fast won’t allow the micro-abrasive to ‘beak down’, Machine linear speed; machine left to right movement shown as inches per second (IPS) apply polish at an MLS of 3-inches per second with a rotary polisher (1.0 to 1.5-inch per second random orbital buffer).

Lessen the Need for Polishing

Polishing removes the paint matrix system's ultra violet protection. Proper washing and drying techniques are the best way to avoid the need for polishing. Using a chemical paint cleanser versus polishing on a clean finish to simply remove old wax/sealant is another. And always follow the rule of using the least abrasive polish and pad combination (working smarter not harder) Light polishes contain either mild abrasives and / or a chemical cleaner (solvent) most mild abrasive polishes will remove medium scratches or surface imperfections

Always be willing to learn; because the more you learn, the more you’ll realize what you don’t know. It is said that knowledge is power, with the caveat that it includes access to a reliable information sources. I would like to think that these articles become an asset to anyone who is new to detailing and to professional’s alike, as well as industry experts who seek to advance their knowledge.

I hope these articles are informative. They are based on the current status of technical development as well as my experience with the products.

By having some understanding of the ‘What’ and ‘Why’ as well as the ‘How’ along with a little science to help you understand how the chemicals we use react, you can achieve the results you desire.

I would appreciate it if you would share these articles as it helps other detailers further their knowledge.

As always if you have questions, I’ll do my best to answer; bear in mind the only stupid questions is the one that was unasked. Questions and/ or constructive comments are always appreciated

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