The most strong strength of the paint film is probably built in the factory one. But the most of the field applied paintings on the vehicles' surface was being built on the relatively dirty surfaces.
In the case of WW2 German vehicles, the front line situation was very imminent the most of the additional painting job were doing on the very dirty surfaces. Then the the strength of the paint film was very weak. Easily chipped and scratched.

Not to mention that most of German field-applied paint was done with very diluted mixes of their paint with fuel, or water (and not always clean drinking water). That combination, applied onto an already dirty surface (maybe not always, but certainly often) would almost certainly lead to a fragile and unstable coat of paint.
As an aside, I'm surprised no one has mentioned the salt grain technique for chipping - mostly over-done in my opinion.

For wear I dry brush. For chipping I do either sponge, hairspray or tiny spots with a super small paint brush

Me too. I've done hairspray method and used chipping fluid but it always seemed over chipped. I like the sponge method the best.

I succumbed to the whole chipping rage as well... back in 1976. I spent several of my formative years in a redneck little burg called Crystal River, Fla, where the SCL ran through. Being a relatively small line at the time, you had to decal and paint most of your own rolling stock. Of of the techniques in Model Railroader was to paint the whole model rust color, and mask off the area with bits of tape, then overspray with body color, in this case black automotive paint. You can see where it worked out okay for a few spots, and where I cheated and just painted the rust on. Fortunately these two spent several days at the little depot in town so I could replicate everything as much as possible. Not bad for little 99 cent Tyco cars if I say so myself. The couplers cost twice that.



Twenty years later (1996) I was still doing the same thing, but using yet another model railroad technique - using a knife to chip the paint off of the rust. It gave much better results. Nothing new with any of these weathering techniques. And just like with landscaping, foliage, structures - model railroading has been leading the way.

Rolling stock (the real stuff) always seem to be scuffed, chipped, and worn to the max. Maybe not a good example to base armor on.

You miss the point. Not the example. The process. It's been around for decades.

Again, we need to separate the 1:1 from a modeling technique. I think this is where we get into trouble and long debates about wether or not “chipping” (an artistic technique) has merit for showing wear on armored vehicles. Agreed, real 1:1 rolling stock spends a lot more time operating on the rails than armored vehicles spend in operation. But is the artistic form of “chipping” any different for model railroaders than it is for armor modelers? I’d think they subscribe to the “more is better” than we would in some cases, where we tend to think “less is better”. But as Robert points out, it’s a universal modeling technique to show a form of wear. And his technique using a knife is interesting. But I think I’d be a little “ham handed” and go a bit too deep with the blade. I have used sanding sticks though, but I prefer the fine point brush method.
VR, Russ

Robert, do you remember when the SCL was the ACL?

You mean ALC and SLC? I remember when they were BNCOC and ANCOC respectively. Now there is MLC as well - Master Leadership Course, required (or not, depending) for E-8.

I think he was referring to this:

The Seaboard Air Line Railroad (reporting mark SAL), which styled itself "The Route of Courteous Service," was an American railroad which existed from April 14, 1900, until July 1, 1967, when it merged with the Atlantic Coast Line Railroad, its longtime rival, to form the Seaboard Coast Line Railroad.

So subsequent to the merger the "ACL" became the "SCL".

Wow, I was pretty off base there eh? I'd heard of the ACL. Don't remember if I ever saw any of their equipment. I did letter a GP-38 for FEC as well.

I made E-6 the same month they made BNCOC mandatory for promotion Sept 92. I was waiting for knee surgery and rehab before I could go. Came down on orders for Field Station Kunia with BNCOC en route and pinning on my E-6 upon graduation with pay and date of rank backdated a year. Went to reup and all NCO's had to talk to I Corp CSM about it. Went to my appointment and got rifted instead. 90 days from my reup date I got a discharge order, had 9+ years in. Was born in an army hospital, dad was a DI at FT. Knox. I was pretty angry.