How thin is too thin?

[Sonic Pilot]

Got a car in for paint correction. As a rule, I always measure the paint thickness on a panel (in several places) before I begin any correction work. In this case the paint measures 2.7 mils in a few different spots. So, how thin can the paint be before the risk of burning or abrading through is just too great?
I'm sure the answer is "it depends on your experience level", but is there a general rule-of-of thumb on how thin is too thin?

[FUNX650]

IMO:
•Concerning oneself about abrading
through the CC is a very important
factor when dealing with any BC/CC
paint system.

•Another factor that may actually be
considered to be more important is:
-the OEMs’ ”rule of thumb”—the one
where they recommend to not remove
more than a grand total of ~0.3-0.5mils
of CC over the expected lifetime of the
vehicle’s paint system.

-By doing so is one sure fire way to
reduce the expected lifetime amount
of the UV protection that’s afforded
by the CC; and, to also void a vehicle’s
Paint Warranty.


Bob

[itsgn]

I'm sure the answer is "it depends on your experience level", but is there a general rule-of-of thumb on how thin is too thin?

Well, everyone has their system and opinion, and of course also everything heavily depends on the actual circumstances, but generally what I'd do is:

1. Measure the thickness of the paint at the door sills around the car.
2. Take the highest value I measured there. If it's below 60 microns (~2.3 mils), I'd take 60 microns instead.
3. Multiply the thickness I got in step 2 with 1.5x.

The resulting paint thickness would be about 2/3 of the thickness of paint at fully clear coated surface, and the thickness I'd never want to go below with any heavily abrasive process, like wet sanding and compounding. (Polishing is a different story, because that only takes 1-2 or two microns at most, so you don't have to watch out that much when doing that. That said, I wouldn't try to polish anything under 80 microns either, unless I - or the person I'm polishing the car for - would be willing to risk compromising the paint to a degree where the car might need a repaint.

Of course this is only mostly true for cars which had an original (untouched) factory clear coat thickness of at least >110-120 microns or more. Some of the newest cars might have a total paint thickness of 100 microns or even less right out of the factory, which of course leaves you with even less wiggle room.

If I'd know that the scratch or scratches I'm trying to remove are likely deeper than what's left of my wiggle room, then I wouldn't even try to go up to (well, down to) the limit calculated above, because it just makes no sense to abrade all the remaining "safely" removable clear coat, when in the end you won't be able to take out the scratch anyway. Then the most you can do is just diminishing it, most of which you can do in the first possibly 10 microns or less. So, I'd only go that far there, even if my calculation would allow for more cc getting abraded.

Also note that if you can measure heavy variances in the paint thickness on a panel (like >5-8% variation) then that panel either has been repainted, was somehow defect already from factory, or has been already extensively spot-corrected, which all would increase the chance of you running over an area that has been already either partially compromised, or where the actual paint thickness could be far less than elsewhere on the panel, even if measurements show it to be pretty high a that spot - because there might be also body fillers or uneven panel surface below the paint. Such a panel would be even more risky to touch, so I'd would possibly add another at least 10 microns to my minimum limit I wouldn't want to go below on that particular panel.

Even then there'd be a slight risk that you're burning through the clear coat or compromising it, just like there always is, under any circumstances, no matter how thick the paint seems to be. So, you always have to "count" with that, regardless of not going with that assumption (ie. that you will burn through at an unexpectedly high remaining thickness already).

[Sonic Pilot]

To "FUNX650",

Bob, thanks for your response. I really like the "manufacturers rule-of-thumb" advice you offered.
Whenever dealing with customers it is always beneficial to have an authoritative and impartial source backing up your advice to them regarding how you can best service their vehicle. Would you happen to have the source for that manufacturers recommendation?

From a practical standpoint (as you also mentioned), attempting to "chase" scratches on paint this thin presents some long term risks like premature paint/clear coat failure that I didn't initially consider. I will be sure to explain this to my client. Thanks again!

[ScottH]

The problem if you are working on a car that is several years old as an example, you don’t have the original paint reading when the car was new. Did it have 5mils? 4? You just don’t know. My rule of thumb is keeping at least 2 mils on the car. Nothing scientific other than it gives me some assurance some clear if left.

This can become a battle with customers who come in for a correction but continue to use the swirl tunnel 51 weeks of the year, and over time your ability to make correction is greatly reduced. Of course you explain this nicely to people, and most will understand but will they change their washing habits?

ScottH

[Paul A.]

GREAT THREAD!!! I would love to hear more from those that know a lot more than me on this subject.

The way I look at PTG values is that they measure TOTAL film build i.e. from substrate up. How much is basecoat.vs. clearcoat is nebulous to me.

For some reason I can't really qualify, 2.0-2.5 would have me nervous and I opt for a cleaner wax/sealant only being very careful of the level of abrasion.

I like itsgn's approach!

[itsgn]

The way I look at PTG values is that they measure TOTAL film build i.e. from substrate up. How much is basecoat.vs. clearcoat is nebulous to me.

So is for everyone else, because only the person who painted the car (or programmed the machine painting the car) could possibly know how much product he has applied, and what the solvent/solid particle ratio of the paint is, etc. Even then slight variances in thickness could and will arise not only between panels, but also between different regions of the same panel.

So, if you're not that person, then you can only make guesses - educated ones at best -, and go by them. Such an educated guess would be that the clear coat layer is thicker than both the base/color coat layer and the primer layer - well, because in most cases it is. In turn you can also make an educated guess that the clear coat is at least half of the overall paint thickness measured over the base substrate (which, however, doesn't mean that it's safe to abrade almost all of it, because it will likely fail a lot "earlier", especially in the long term).

Of course because these are still just guesses, it could turn out that you were wrong with them, and the ratio of clear coat / base coat is different. In which case you could burn through the clear coat or not correct an otherwise correctable scratch. Obviously the latter is usually a far less severe consequence than the former, so it makes sense to always opt to be on the safe side.

That said there are paint gauges that can measure the thickness of multiple substance layers separately, and can actually tell you the thickness of the clear coat layer only, not counting everything below it. They can also work on plastics, unlike the regular ones, which only work on metal panels. However, these are obviously very expensive tools, costing well into the 4 figures, which will never pay for themselves for an everyday detailer as long as he's using some common sense and just avoids jobs or attempted fixes that are too risky.

[Paul A.]

Thanks, itsgn...good info.