Kinetic friction induced heat is an often misunderstood concept of polishing / compounding; abrasives require friction to breakdown, not heat; heat is just a resultant of friction between two surfaces.



Pads - Wool vs. Foam



Pro -faster cutting generates less friction induced heat and therefore leaves the surface cooler, removes heavy oxidation much more efficiently than a form pad. Use foam pad for slower and finer cutting. Wool pads are easier to control compared to foam. They glide as the wool nap ‘lies down’ and will conform to vehicle body shape easily. Make certain to spur the wool fibres when it loads up with product.



Con -wool can leave micro marring (buffer trails) finish with a foam pad to remove. Don’t overwork the polish as wool will break diminishing abrasives faster than foam, the surface lubricants will dry up faster and you could end up dry buffing unless you’re careful.

There are too many variables with different wool and foam types, along with too many compounds to give a definite choice.



Kinetic (or dynamic) Friction



Polishing a paint surfaces transfer’s kinetic (or dynamic) friction induced heat to the paint surface, thermoplastic polymers have both tensile strength and elongation (elasticity) which allow the surface to flex, expand and contract in accordance to surrounding temperatures, solvents, resins and other ingredients in polishes will expand causing the paint film surface to expand.



As the metal substrate expands the paint moves with it, due to its elasticity, thereby becoming elongated (thinner) this is part of the cause of friction induced ‘burn’, you’re applying pressure and an abrasive to a less dense (‘thinner’) paint surface.

The smaller the contact or surface area of the pad the more kinetic (heat) energy is transferred to a given paint surface area. (Area = π (r2) - 6-inch = 28.26 sq.ins - 4-inch = 12.46 sq.ins. The amount of applied pressure will also affect the kinetic friction (heat) energy transferred



Heat from Kinetic (or dynamic) Friction



[Energy in a system may take on various forms (e.g. kinetic, potential, heat, light). Kinetic friction, or surface resistance induced heat; an often misunderstood concept of polishing / compounding; abrasives require friction to breakdown, not heat; heat is just a resultant of friction between two surfaces. Kinetic friction is required to ‘level’ paint, which is simply the removal of paint to the lowest point of the paint defect] [1]



Polishing a paint surfaces transfer’s kinetic friction induced heat to the paint surface, thermoplastic polymers have both tensile strength (a linear stress-strain relationship) and elongation (elasticity) which allow the surface to flex, expand and contract in accordance to surrounding temperatures, solvents, resins and other ingredients in polishes will expand causing the paint film surface to expand



As the metal substrate expands the paint moves with it, due to its elasticity, thereby becoming elongated (thinner) this is part of the cause of friction induced ‘burn’, you’re applying pressure and an abrasive to a less dense (‘thinner’) paint surface, excess friction induced heat can cause the paint surface to burn, blister, haze, and cause excessive swirls.



Plastic has a much lower rate of thermal conductivity than metal, so it absorbs heats at a far greater rate and therefore is subject to localized ‘spot’ heating.



Polishes and compounds do not need heat per se for the abrasives to polish a surface, wither they be diminishing or non-diminishing abrasive, they require both pressure and friction Kinetic friction induced heat can cause a rapid temperature rise; (i.e. initial surface temp 80.oF, friction heat attained with the polisher stationary and a cutting foam pad at 1,100 RPM for approx. ten seconds the friction induced heat attained would be around ~104.oF)



In accordance with the Society of Automotive Engineers (SAE) a localized (spot) temperature should be limited to 115.oF (range 105 – 145.OF) Above these temperatures and dependent upon paint specification, thickness, and etc) it may cause the paint to soften and the resin binder system to eventually fail



Above 115.O F and you create what the coating industry calls "thermal stress" - those long, small cracks in the paint that looks like light scratches, but cannot be removed by buffing. This phenomenon won`t show up at first, but in a year or so the cracks will be evident. (See also the first law of thermodynamics et al) When a localized spot is significantly hotter that the surrounding area you have a potential problem, the paint temperature can be checked by utilizing an instant read-out infra-red ‘gun type’ digital thermometer



A finishing pad will not provide as much friction as a cutting foam pad (less surface resistance) although they will both produce friction induced heat, whereas a wool pad, due to their composition, creates less friction induced heat but more kinetic friction (due to its fibrous structure) than most foam pads. Be cognizant that with high ‘spot’ temperatures the pad structure will cause scratching that is forced deep into the clear coat.



You should try to maintain as cool a surface temperature as possible when polishing, use your hand to provide a general idea of how much heat is being transferred by kinetic (friction) energy; it will of course be at a higher temperature than ambient. An orbital polisher tends to concentrate higher temperatures at the centre of the pad, whereas a rotary the heat builds up more toward the outer edge of the pad.



Applied Pressure



The pad needs to have an even distribution of pressure applied to it; depending on the types of surface abrasions you`re dealing with, increase pressure as necessary. Just remember that more pressure equals more aggressive, so be careful around ridges and raised surfaces



Maintain the same pressure and work the product in, it may take three or four passes to complete before the residue can be removed. Once you see the desired results move on to the next area, or repeat the process as necessary.



The required pressure applied to obtain optimum results to adequately compress the pad (50%) and obtain uniform abrasion is usually in the range of 10 – 15 lbs. (a random orbital buffer will stall at approximately 20 pounds of applied force) To compress a 6-inch pad 50% requires you increase the total force by the ratio of its surface areas Ratio = [π (radius2)] / [π (radius2)] = 2.25 as much force, almost 34 psi).



With the smaller pad you`re applying the same force, at a constant speed but over a smaller, more concentrated area, which will induce an increase in friction and greater abrasion abilities to the polish / pad combination, both these abilities require a certain amount of caution as it’s possible to ‘strike through’ (friction burn) the paint.



Trailing and Leading Edge



Looking at a pad that is on the paint surface; the trailing edge is the left side (between 6 and 8 o’clock) and the leading edge is the right side (between 1 and 3 o’clock)



A light sensitive approach is essential, using only the weight of the machine, on vertical panels just enough pressure to maintain contact with the surface (without applied pressure) use a lower speed 1000 – 1200 RPM and keep the pad moving.



Tilt the contact edge of the pad a few degrees so that only the leading edge of the pad is in contact with the paint surface. Make sure the leading edge (right hand side) of the pad on a rotary polisher is rolling off the panel so that would mean the right side of the pad is rotating off the panel.



If you have the trailing edge (left side) of the buffer on the edge it will tend to strongly force the machine out of your hands and burn the edge almost instantly.



Strikethrough



Strikethrough and Paint burn have very similar cause and effect and these terms are often used interchangeably. It takes longer to strikethrough paint in the middle of a panel as opposed to an edge. Basically strikethrough is caused when kinetic energy (friction heat) has compromised the clear coat and exposed the base coat. Paint often looks a slightly lighter colour; it’s usually concentrated on a small area or ‘spot ‘



You will also be able to see a non-glossy patch (base coat) where the clear has been removed with the edges of the remaining clear coat being visible. Since the clear coat contains the UV inhibitors, the colour coat is now exposed and will be subjected to photosynthesis (fading)



• Speed - using too high a speed will not necessarily get the job done faster as there is a risk of instilling swirl marks or strikethrough, which will need to be corrected to remove



• Pressure - excessive pressure will make the pad / polish combination more aggressive, this has the effect of increasing kinetic energy (friction heat) which may result in a strikethrough, a friction paint burn or paint delamination from the substrate. Increased surface friction will also cause swirl marks



• Heat - excessive friction induced heat and a combination of excessive pressure (surface resistance) speed and an aggressive pad / polish combination will rapidly generate surface heat, this will soften the paint and may cause delamination from the substrate, surface hazing, strikethrough and greatly increase the chance of swirls.



• Pad angle – relative to the paint surface ideally the pad should be operated flat to the surface; this provides the correct contact surface area along with sufficient surface lubrication from the polish oils. By turning a pad on an angle you reduce the surface area contact, increasing pressure and kinetic energy (heat) while, reducing the amount of surface lubrication available.



• Insufficient product - without the polish lubrication oils, dry buffing will cause delamination from the substrate, surface hazing, strikethrough and greatly increase the chance of swirls.



Applying pressure and holding the polisher stationary in one area for too long will cause friction heat to be concentrated on a small area, especially with a foam pad due to their high surface resistance (friction heat) although a wool pad has a much lower surface resistance they are more abrasive.



Paint Burn



Paint burn and Strikethrough have very similar cause and effect and these terms are often used interchangeably. Urethane clear coat uses a catalyst (hardener) if you heat it to beyond its melting point it will fracture and result in ‘paint burn’.



Although it is usually a combination of things, the most common cause of paint burn is thin paint (on an edge or end panel) a lack of surface lubrication (dry pad) and kinetic energy (friction) causing the pad to becomes more like a sanding disc, which produces excess surface heat, consequently the paint reaches its melting point and fractures



Clear coat provides both ultra violet (UV) radiation and the paint systems protection. Repair any breaches in the clear coat system otherwise you risk the paint drying out and delaminating, which will then require repainting



Areas of risk



Bumpers - often made of a composite material, they don’t conduct heat like metal panels, the paint in these areas, especially edges are generally thin (masking tape is good insurance)



Panel edges and seams - when approaching an edge always make sure the pad backing plate is spinning from the panel toward the edge and not coming from the outside to the edge where it can catch the edge and strikethrough.