Painting Prep for Cars: Common Auto Body Paint and Bare Metal Mistakes to Avoid

How to Avoid Common Car Paint and Bare Metal Prep Mistakes

Manufacturers' technical support personnel often field calls from body shop employees about product failures that lead to poor paint jobs - only to find out it wasn't the products but faulty paint prep procedures.

Paint and paint preparation product manufacturers have heard it before. A call comes into the technical support center from a painter or body technician, and he or she says, “Hey, your product didn’t work…” or something along those lines. The final paint job had some sort of flaw, and usually the end user blames the product.

According to the manufacturers, however, the problem is usually not the product but how it was applied. Whether it has to do with painting, sanding or applying body filler, sometimes the technician didn’t read the directions, or maybe he or she inadvertently contaminated a panel.

Most of the manufacturers have enough stories like these to fill a book. And all of them teach some very important lessons. Here are some of them:

Filter and Adapt

Dave Gzik
PPS Technical Service

The most common calls we receive concern adapters. The painter has been using the 3M PPS System, just bought a new gun, and the jobber couldn’t supply them with the proper adapter or tell them what adapter number to use on the gun.

The second most common call concerns waterborne. They say that they heard there is a different filter they’re supposed to use in the lid. Of course, our lids come with prewelded (sonically welded) filters. For waterborne, most paint companies recommend a 125-micron or finer mesh versus the standard 200-micron.

The Case of the Mysterious Handprint

Jimmy Castillo
Regional Training Instructor

Many times, collision center prep departments neglect the cleaning part of the refinishing process.
I encourage technicians to wear gloves while prepping, but most don’t like to because their hands end up sweating. But it’s all about developing good working habits and cleaning panels before actually sanding them so contamination is removed.

I’ve seen it one more than once where they’re prepping a panel that’s stripped down to bare metal and they’re looking for leverage, so they lean on the panel while sanding another part of it. If they don’t go back and clean the panel where they were leaning before applying etching material, primer surfacers and eventually basecoat and clearcoat, they’ll leave behind salt and oil from the skin.

In as little as two months, you can actually see the imprint of a complete hand or wrist coming from underneath the paint. Sometimes it shows up in the form of small blisters, depending on what part of the panel they were touching and what part of the hand was touching – sometimes the finger, sometimes the whole hand.

Surface prep is extremely important to what happens at the end. Everyone loves to see a shiny, nice-looking paint job, but how you get there starts in the beginning.

Techs should wear gloves. We get used to doing things a certain way and we learn how to do something from someone else. If we pick up bad habits through the years, then we maintain those bad habits.


Peter Mahoney
National Training Instructor
Chemspec USA

In my experience, when a vehicle first comes in, many shops fail to wash it down properly with wax and grease remover. When they don’t do this, multiple things can happen.

I recently went to a body shop that told me our primer had failed and that a giant blister had formed in a concave area of a GM Silverado.

I put some duct tape on the blister to pop it off in order to see the backside of the paint film. When you do this, the primer lets loose from the metal, and when you look at the metal, you can see if it was sanded properly. I could tell the metal had been sanded properly, but when I looked at the primer film on the back, there was no sand scratch, which told me there was a contamination on the surface filling the sand scratch. I attributed that to not washing the panel down before they started the work.

With the waxes and teflon coatings they have today, you have to do things differently. When I was a painter, my mentality was, “I’m not washing it down because I’m going to sand it all off.” The problem with that thinking is that with that concave area, you’re really hitting the DA in that area. If you don’t change your DA disc frequently, you’ll end up featheredging the area, but basically you’re getting wax or the teflon coating on your disc. The heat generated then melts the wax or teflon into the sand scratch on the metal. Then, you end up putting the contaminant on the metal surface. You won’t notice that when you’re spraying, but you will notice it down the road, whether it’s three weeks or one or two months.

Make It Stick

Jim Evans
National Training Manager

The other day, a customer called to say that the paint and clearcoat had come right off the surface of a raw plastic polyolefin part after he accidentally flexed it as he was carrying it to the car for installation. As usual, it was the last piece he was putting on the car and the car had to go out later that day.

Our tech called him and asked what steps he took to prepare the part for paint. He said he first cleaned it off with soap and water. Good. He followed it up with 222S adhesion promoter, and as soon as he said that, the tech knew that was the first problem. He then tacked it off, put his basecoat and clearcoat down with a 10-minute flash time in between, and then baked it.

The 222S adhesion promoter is not made for plastics. He should have used Plas-Stick 2320S. The directions are to saturate a cloth with it and clean the plastic thoroughly until all the gloss is removed and it doesn’t feel slick. After applying the Plas-Stick, you either hand sand the plastic with 800 grit or DA it with 500. Then you clean it again with Plas-Stick to not only take the sanding dust off but minimize static build-up. Once you reclean, you apply one more medium coat of Plas-Stick and allow it to dry. If you’re going to prime it, you would put Plas-Stick in the primer to flex it and put it in the sealer if you’re going to seal it. Then you would apply your basecoat and clearcoat. If you do that, there won’t be any issues.

Crack Me Up

Ed Medina
Zone Manager

The most common issue I’ve seen is under catalyzation of body filler or putty. The shop will call and say, “Your filler is cracking.” We analyze the dry product to determine how much BPO cream hardener was in it, and generally it was less than half a percent. We’re trying to break shops of that habit with newer products like our Quantum1, which has multiple catalysts and allows them to mix it the same way every time without having to increase or decrease the amount of hardener for different temperatures.

Another issue we see is solvent absorption, which results in solvent popping. At what point does body filler and/or putty become impervious to solvent absorption from primers and topcoats? Generally, it’s 45 minutes for conventional filler and putty. A lot of shops don’t know that and are trying to rush jobs through. In the past, body shops were inclined to put primer on everything that had come from the shop that evening so it would fully cure and be sandable in the morning. So we had to educate the industry on that. Now, with Quantum1, that 45-minute wait time has gone down to 19 minutes.

Have a Cocktail

Craig Chaffee
Product Development Engineer
Norton Abrasives Products

The biggest problem I see is guys not reading the directions. I’ve even gotten calls from people who use an adhesion promoter when they’re not supposed to. For instance, using an adhesion promoter that’s a structural adhesive for a bumper, not paint. They just see the aerosol spray and say, “Hey, it’s an adhesion promoter.”

There are even issues with scuff gel. One time, this guy was using a certain paint manufacturer’s scuff gel but wanted to switch to an off-the-market one. I said, “I can understand you trying to save money, but what happens when you have a warranty issue that paint manufacturer won’t back because you didn’t use the specific scuff gel within the product line?”

Even after the refinish process, you have people “cocktailing” products who end up getting cross contamination between someone’s compound and someone else’s pad and then wonder why things didn’t go right. Mixing different product lines, especially in the paint world, can be bad.

Fading Away

Steve Lanyard
Product Manager

I get a lot of calls about color match. The callers are trying to paint a single panel on a 10-year-old car where the overall paint has faded, and they don’t consider that they have to blend. They don’t want to blend, they want to paint just the panel, but we tell them you have to blend. They expect to paint a quarter panel and it will match the door or paint a fender and it will match the hood. They have to understand the car has been sitting in the sun for 10 years. They could be a hobbyist or a new person who doesn’t have the necessary experience under their belt.

Paint Finish from Scratch

Bill Warner
Technical Manager
Pro-Spray Automotive Finishes

I hear a lot of people complaining about getting sand scratches in their finish under the clearcoat. I tell them that all you have to do is move to a finer grit sandpaper.

They want to prepare panels with coarser sandpaper because it appears to cut faster, which it does because it’s more aggressive. But it’s also cutting deeper, and the guys don’t equate to the deeper part. So they try to solve the issue by spending more money on colorless basecoat, or color blender as we call it. They’ve been taught this by the paint companies. But I tell them you don’t need it if you move to a finer grit sandpaper because it gives you more surface area than a coarser grit sandpaper does.

There is actually more little abrasive minerals on that 800 grit sheet than the 180, but they’re spread out more. Therefore, it leaves a coarser and deeper scratch, but you don’t have as big a tooth or surface area covered with sand scratches. But guys think if they move to a finer grit, they’ll lose adhesion. That’s when I tell them my recommendation: anything from 1200 to 1500 grit, but realistically you could probably use 1000.

Getting All the Facts

Randy Schneider
Marketing Manager
Rubber Seal

RS-790 Primer Surfacer is not a direct-to-metal primer. It can be applied to bare metal only on sand-through areas about the size of a silver dollar.

A customer had a product issue where the primer was lifting after applying two coats of basecoat. The salesman questioned the painter about the process he used. After gathering all the information, I duplicated the entire process with no problems. The product performed fine.

Weeks later, I found out the customer added an accelerator to the primer surfacer, which is fine. However, the customer added it at a rate of 25 percent of the ready-to-spray product, when it should have been 5 to 10 percent. That’s what caused the product failure.

This is a good example of not getting all the facts when trying to solve product issues.

Primed vs. Unprimed

Kurt Lammon
Urethane Supply

Almost all of the Toyota and Nissan bumpers come from the factory unprimed. Most domestics come primed. However, looks can be deceiving. Sometimes the primed bumpers look unprimed, and that’s where our customers sometimes get in trouble.

The call usually goes something like this: “Yes, I wanted to let you know that I used your Bumper & Cladding Adhesion Primer on a Chevy Cobalt bumper, and it came back peeling off in sheets.”

I reply, “You know that the primer is only for RAW polypropylene and TPO bumpers, right? Cobalt bumpers, like most domestic bumpers, are primed. We don’t recommend our primer on primed bumpers.”

Then I’ll explain that they need to use a light-colored, fine sandpaper and lightly sand the bottom edge of the bumper to see if it dusts up. If any sanding dust is generated, then it’s a primed bumper. It’s still the technician’s responsibility to determine if the substrate is raw or not.

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