Glad you’re enjoying the articles and thanks for reading them! let’s see what we can do to help or at least provide the answer to your first question, “Is my dad right (that you need heat)?”, is simple: No. There is some thought that when polishing older nitro-cellulose paints, like lacquer, you could heat the surface enough to move it around a bit. This heat might have helped smooth orange peel or push some material (paint) into a scratch. However, I have never seen this happen, and when I have spoken with paint chemists they refute these claims as well.
Modern paint is chemically cross-linked. I am not a chemist, but I have been fortunate enough to work with many. There is a chemical process, that when completed properly, renders the paint cured. The idea of “reflowing” the clear coat, once it has cured, would mean breaking the chemical bonds (through temperature) and then somehow reestablishing them by cooling. It is not my opinion that this is or isn’t possible, but rather the opinion of the paint chemists and chemical companies that this is impossible, based on their factual knowledge, testing, and experience. Once the chemical reaction occurs and the paint is cured it cannot return to a liquid state; at least not one that would then reharden again.
I have had the guilty of pleasure of purposefully damaging paint with heat. I have superheated the paint to the point where it has become soft and I could print my thumb into it, where the clear coat has detached from the base coat, where the paint becomes smooth only to cool off and have scratches return (more on this later), where it becomes stained from the heat, and where it is literally steaming, yet I have never seen it turn to liquid. However, paint does expand when subjected to heat. Paint has to be elastic to a degree in order to survive what it is exposed to! If it didn’t expand, contract, or flex it would crack when the underlying panel expanded in the afternoon heat, or as your car bounced down a rough road, or that stray soccer ball from the neighbors hard bounced off your hood by accident. Cars are sturdy, but they are not completely rigid. Panels flex, contract and expand and so too must the paint that is on top of them.
To illustrate how paint behaves when it swells from heat picture a deflated balloon. It looks wrinkled and bunched-up, but as we fill the balloon with air and it SWELLS the surface stretches out and the wrinkles disappear. This is, in a way, is similar to how scratches disappear when paint swells. As the paint gets hot and expands it stretches the scratches out and makes the surface smooth. Just like if we let the air out of the balloon it would return to a wrinkled appearance, so too will paint when it cools and shrinks.
You mentioned that your father and yourself are removing sanding scratches in a bodyshop. I am going to assume the paint is fresh, or close to fresh in most cases, and you are sanding to either remove orange peel, nibs, or other imperfections. Working on fresh or recently sprayed paint creates a new challenge because the paint might still have some solvent trapped underneath. When the paint is sanded and then heat is generated by polishing, the trapped solvent can also make the paint swell. Now you have two sources of swelling – chemical swelling and heat swelling. It is the combination of both that helps the old school body guys maintain the myth that you need heat to remove scratches. They polish and heat the top layer of paint, it looks great (this effect can last days and in some cases weeks), and they decide heat is necessary to “flow” the clear…
This is all just a long way of saying your dad has been working with some incorrect information and heating the paint up not only isn’t necessary, but it can temporarily hide defects that lead to the scratches returning later.
To answer your second question, “why are you fixing your dad’s work?”
I would start with what I explained above. Your father is likely putting a lot of heat into the clear during his initial polishing process (thus thinking heat is beneficial) and then when the paint settles back down you are left to fix the clear coat. One of the advantages of the RUPES BigFoot Random Orbital Polishing Systems is that they heat the paint less than other paint systems, which means less risk of heat swelling. Although it may appear to take longer to remove the scratches, the end result is it’s faster because you don’t have to go back and address them again when they return. What you see is much closer to what you get.
There are a few things you do to help combat the chemical swelling issue. Spend more time sanding, and sand to finer grit level. Sanding (especially when done wet) allows us to change the profile of the paint and refine the surface with the most minimal rise in temperature. A finer scratch also requires less aggressive polishing and thus less of the unwanted by-product of friction in a more aggressive polishing process.
Follow the paint tech sheets to a tee. Spray the correct amount of base coat and clear coat, with the appropriate amount of dry time between each coat. If you over apply the paint (very common) you can trap a lot of the solvent underneath which has a hard time escaping. Just remember that each time you polish you get more and more potential swelling. Most tech sheets also have a recommended “polishing window” that define a range of hours from the time the paint is sprayed that it is recommended to be sanded and/or polished – do your best to sand and polish in this timeframe! Some paints become incredibly difficult to polish after the “buffing window” passes.
Hope this helps and clears up any confusion and doesn’t create any new arguments between you and your father. The world of paint has become increasingly complicated over the years and he is not alone in his assumptions about heat, but at least he has an enthusiastic son by his side to help educate him.
Yours in better polishing,
Todd Helme | Senior Technical Advisor, RUPES USA