In the third installment of my blog, I’ll look at my training regarding polishing and offer my thoughts on this massive topic. This could be lengthy…
The initial points that Rich and I discussed were basic, but possibly much overlooked: what exactly is car paint and, crucially, how much paint depth do cars typically have? I was aware that total depth is usually pretty thin, but only when you have a laboratory reference shim in your hand do you realise that 100 microns is not a lot. For reference, a typical sheet of paper is around 70 microns thick.
Paint defects generally comprise any damage that has broken the top layer of paint (usually termed the clearcoat) and damage such as stone chips and swirl marks are probably the most common. For the purposes of this blog, I’ll refer to any abrasive product as a polish and any non-abrasive product as a glaze in the following discussion.
In a nutshell, polishing removes some of the clearcoat and essentially lowers and flattens the finish, thus removing any paint defects present. This point is vital as polishing will always remove a fraction of the clearcoat. Polish too often and too much clearcoat will be removed, resulting in paint system failure. It’s generally accepted that removal of up to 25 % of the clearcoat is safe over the lifetime of the average car. Paints vary in terms of hardness and resistance to scratches and chips, and car manufacturers select paint type based on the properties they place most importance upon.
An alternative to polishing out fine swirl marks and other minor defects is to use a glaze. Imagine the paint finish sideways on. The defects will look like valleys. Glazes work by filling these valleys up with oils and clay particles, which reduces the visibility of defects. As glazes are non-abrasive, they do not remove any paint. The downsides of using glazes are: (a) defects are only hidden, not removed, and; (b) the effect is temporary, because the fillers will erode away over time, re-revealing the original defect. It should be noted that glazes must always be topped with good sealant or wax protection, otherwise they will be quickly washed off.
Polishes, as I’ve mentioned, remove paint and, in doing so, will remove defects. A polish will contain very finely milled abrasive particles, generally of aluminium oxide, which are worked across the paint surface to remove some of the clearcoat. In my mind, their mode of operation is very similar to how a sponge works (or doesn’t!) on paint; cleaning a car with a sponge results in particles of dirt being trapped between the paint surface and the face of the sponge. This induces swirls and scratches.
Polishing, in my head at least, works in a similar way: the particles in the polish are trapped between the paint surface and the polishing pad and worked across the paint. The important difference is, the polishing particles are very finely milled and are designed to be used to cut the paint surface. When polishing, I find it useful to think about what is actually taking place and in my, perhaps slightly odd, mind, I imagine a microscopic image of the particles being ground into the paint surface and some of it being removed (sometimes I actually imagine each particle is a Pac-Man, eating tiny pieces of the paint surface – I also think I shouldn’t be allowed out in public).
Polishes contain oils and other chemicals to ease their operation. Many polishes utilise diminishing abrasives, meaning the particles will break down when they are worked into the paint. The smaller they get, in theory, the more refined the finish will be. Most polishes start to turn see through when they have been worked sufficiently and this occurs when the diminishing particles have, unsurprisingly, diminished.
More aggressive polishes can contain a higher proportion of abrasives or larger particles which take longer to break down and offer a higher level of cut. They will also, obviously, remove much more clearcoat than a less aggressive polish. Some may contain microscopic particles that do not diminish and will simply continue to abrade for as long as they are worked (up to a point). This type of non-diminishing particle offers differing results based on speed of work, pressure and pad type.
A further variable to consider is the choice of pad. A very soft pad working a very aggressive polish is not an advisable combination as the pad will not force the particles against the paint with sufficient pressure. However, a soft pad is ideal for use with a finishing polish as its softer nature will offer less cut and will allow the finishing polish to bring out a superior level of shine.
A very stiff pad is ideal for working an aggressive polishing compound as it will allow more pressure to be applied, forcing the polish to cut the paint harder. However, a very stiff pad will tend to have an abrasive texture and simply working the pad itself into the paint will lead to some cutting effect. Therefore, stiffs pads are not well suited to producing exceptional paint finishes and are generally not recommended for cars with soft paint.
Pad selection is something of a minefield, made more difficult because each manufacturer tends to grade and name their pads in slightly different ways. For example, comparing polishing pads from two different manufacturers is not necessarily comparing like with like.
The principles of polishing are the same whether working by hand or machine. Working by machine, assuming guidelines are properly followed, will result in faster results and probably a higher level of defect removal as polishes can be worked harder and faster than is possible by hand. However, polishing by hand can still bring about excellent results on soft and intermediate paints, but it will be much harder work.
During the hands-on part of my training, I tried working various glazes into an old scrap BMW panel both by hand and machine to see (a) how much defect masking is possible, and (b) whether there is any difference working them by hand and machine. I then went on to test various polishes from very fine finishing polishes through to very aggressive cutting compounds, by hand and then by machine.
The glazes, on the whole, did a reasonable job of filling lighter defects and are thus ideal for masking lighter wash marring. Some worked more effectively by machine, while for others, using a machine made little difference compared to hand use. All left a noticeably smoother paint finish, probably as a result of the glazing oils contained within them.
The finishing polishes, worked by hand, didn’t remove a tremendous amount of defects but there was a noticeable difference in the paint’s appearance. Using the more aggressive polishes by hand led to some interesting results: all removed defects to varying degrees, but interestingly one of the most aggressive products, which I suspected could not be worked hard enough by hand, yielded excellent results.
Working the same polishes by machine led to a similar pattern of results but the defect removal was greater and the working time was much reduced – plus my arms were happier! The most aggressive polish, best described as liquid sandpaper, removed all but the deepest of scratches but did leave its own light scratches; the abrasives were so strong that they themselves marked the paint even when the polish was worked fully and all particles were broken down. This micromarring effect was easily removed using a finishing polish.
The next stage, and one which I was rather unsure about, was wetsanding. After a demo from Rich, I stuck a 2000 grit Mirka Abralon disc onto the polisher and started some serious clearcoat removal. I worked the 2000 grit disc, then refined with a 4000 grit disc and finally switched to Meguiar’s’ #105 to remove the matt-looking haze. The benefit of using the Abralon discs is that the material features uniform abrasives: sandpaper tends to have some naturally larger particles which will inflict sanding scratches. After refining with #105, almost all the scratches were removed: the ones that remained were exceptionally deep and had probably penetrated the total depth of the clearcoat.
The paint on the scrap panel was particularly hard and deep (being older it was solvent-based), but despite this, the amount of clearcoat removal was in the range 5-8 microns. Modern, water-based paints, and softer paints, would have seen an even higher amount of paint removed.
To sum up then, my very own golden rules of polishing:
• Don’t polish until you absolutely have to – consider glazing instead;
• Pick the least aggressive polish required to get the job done and minimise paint removal;
• Select your pad carefully, whether working by hand or machine.
My next article will be about paint protection and will be published next month, just in time for the onset of the better spring weather. As always, if you have any questions or comments please feel free to fire away.