You gotta prime it quick after blasting so it doesn't rust. If you take time to do the filler work while the body is bare... rust. Regular polyester primer absorbs moisture. If that's all that's on it while you do the filler work... rust.
Have you ever popped a chunk of bondo off a fender and looked at the metal where it was adhered? It is rusty. It wasn't when the bondo was applied likely. Someone sanded that area down and spooned in some body filler. As it hardened, the body filler absorbed atmospheric moisture from the shop, dried from the surface in, trapping that moisture and the fender rusts, sight unseen. Over epoxy primer, the same process occurs but the moisture is denied bare steel due to the primer. Problem solved. Both processes will work, but on my cars, they are body worked, epoxy primered, body filler, sanded, polyester primer or high build, blocked, sealed and painted.
Have your primer and hardener measured out, but not mixed before, and your gun clean and ready. After you clean your metal surgically clean, and when you are waiting for the cleaner to flash off, mix your primer and fill the gun. Spray immediately. Do not wait. If you have inclusions, drips, sags, etc., do NOT stop. Finish the coverage. Sand the errors out when the primer has cured. Recoat as per product instructions for a next coat.
I'm in the "filler over bare metal" camp, and I used to be a rep for Evercoat. I had a lot of conversations with John Hatton who developed the resin technology of today's fillers. His resin formulas revolutionized modern body fillers and gave autobody fillers exceptionally advanced adhesion and workability. I asked him about filler over epoxy, and he said it will adhere fine. In fact, his resin technology will adhere to good, fully cured, 2K primer and paint. (Have you tried getting cured filler off your fingers?) I see lots of posts about putting filler over epoxy, but very seldom is the epoxy product brand/name mentioned or the cure time that is recommended and prep (sanding/scuffing) before the filler is applied. There are so many different "epoxy" primers, and they are not all the same. The biggest problem is applying epoxy OR filler over metal with a small amount of humidity/ moisture on the surface of the metal. I'm in the northern USA and it's usually cold enough that I'm more cafeful about metal temps when I do filler or primer. Surface temps of the metal can easily be 10-15 degrees F cooler than the air temp in the shop this time of year. When you take a shower, the mirror "fogs" up in the bathroom because it's colder than the air and moisture condenses on the cool surface. If you warm the mirror with a hair drier, it won't condense the moisture in the air on the warm mirror. This is an exaggerated example of what happens on the metal/surface of your filler area even with a small amount of humidity in the air. I live in a northern climate where temperatures range from -35F to over 100F, so I try to pay close attention to shop air temp and the actual temp of the metal when I'm getting ready to do filler work. Most of the year it's cooler and I have gotten into the habit of warming the panel(s) with a heat lamp to get the metal temp up to at least 70-75 degrees F before applying primer or filler, and too much heat is not good either. I'm looking at 2 things when I pre warm the panel: I want to remove that slight layer of humidity on the cooler metal, and I want the filler to cure more evenly. Body filler (polyester resin) stops curing below 64 degrees F, and can cure too fast and the resin can crystalize over 90 degrees F. If your panel is much cooler than the air temp, it can cure from the outside in trapping the moisture under it, and also trapping gasses (air pockets) and solvents (styrene) in the filler layer. I'm sure everyone who works with body filler has found rusty surface under an old repair, usually from moisture on the surface of the steel and pinholes, sometimes caused from too fast curing. The mixing ratio of hardner to filler is also a factor and the filler can easily be over or under catalyzed. That's why the temperature is so critical when using filler, so you can keep the curing process optimum for gas and solvent escape. If the air/shop temp is over 90F I'm usually not going to do filler work until it cools down some, and certainly not in the direct sunlight. You can only reduce the catalyst/hardner amount so much without risking that the filler won't cure completely and too much hardner will cause the filler to cure to quickly and become brittle and trap gas bubbles in the mix. Here's basic info from the internet: The best curing temperature for most body fillers is around 70 degrees Fahrenheit (21 degrees Celsius), with an ideal working range between 60-90 degrees Fahrenheit, as colder temperatures significantly slow down the curing process while extreme heat can cause the filler to set too quickly. Key points about body filler curing temperature: Optimal range: Most body filler manufacturers recommend working within a temperature range of 60-90 degrees Fahrenheit. Cold weather impact: If the temperature is too low, the filler will take much longer to cure and may not fully harden. Heat impact: While heat can speed up curing, excessive heat can cause the filler to set too quickly, making it difficult to work with. Check product instructions: Always consult the specific instructions on your body filler product for the recommended curing temperature.
I got my whole car to bare metal after doing all of the patch panels, and dent removal. Then shot epoxy primer/sealer, and after that I did the final filler to get the last of the bodywork better. Followed that up with high build primer, and my paint.
Filler is porous. If moisture gets in (or is trapped in after wet sanding) bare metal under it WILL rust. Put Filler over epoxy primer and it will protect the metal.
Which epoxy primers are completely water proof? Epoxy primer does NOTHING to neutralize rust. It covers metal and rust. How do you prep the rusty metal before applying epoxy primer?
Is it really E-coat? What is the process for Electrocoating? EDP Electro deposition primer (E-coat) In the first step – pretreatment – metal is cleaned and a phosphate is applied to prepare the surface for application of the e-coat. This process is essential to achieving the performance requirements desired by product end users. PPG Coatings Services analyzes the metals to be processed and chooses the most appropriate chemicals for cleaning and phosphating. In our facilities, the most common pretreatment regimen for steel and iron parts is immersion in a high-quality zinc-phosphate system. During the next step, coatings are applied to the pretreated metal in an electrocoat bath using precisely calibrated process control equipment. The e-coat bath consists of 80-90% deionized water and 10-20% paint solids. The deionized water acts as a carrier for the paint solids, which are under constant agitation. The solids consist of resin – the backbone of the final paint film, which provides corrosion protection, durability and toughness – and pigments, which contribute color and gloss. Post-rinsing, which occurs next, enhances the quality of the coating and enables the recovery of excess paint. During the e-coat process, paint is applied to a part using a regulated amount of voltage to achieve the desired film thickness. Once the coating reaches the prescribed thickness, the part “insulates” and the coating process slows down. As the part exits the bath, paint solids that cling to the surface are rinsed off to maintain application efficiency and aesthetics. This residue material, which is called “drag out” or “cream coat,” is returned to the tank, enabling the e-coat process to achieve application efficiency rates above 95%. After the coated part exits the post-rinse phase, it is placed in a bake oven that cures and cross-links the paint film to maximize its performance properties. The minimum bake time is 20 minutes with a part temperature at 375°F for most e-coat technologies. PPG Coatings Services can also apply a supplemental “low temperature cure” e-coat material depended on volume. Most, if not all, OEM automotive manufacturers use this process, or similar, for replacement parts--aftermarket parts manufacturers, probably not. I emailed (about 4 years ago)a large aftermarket importer of automotive replacement parts and asked if the primer was an actual electro deposition primer and if the steel was phosphated before the process. Surprisingly, about a week later I did get an answer, that the steel was cold rolled steel with no phosphate coating and the primer was just a "transport" primer coating. If you have noticed pits or rusting under the primer on aftermarket parts, those parts were probably exposed to humid conditions before priming and shipping and never had a phosphate coating. As for me, I will strip the primer/coating and probably metal etch or at least etch prime the panel before proceeding with my primer/paint system. If there is rust under the primer it is NOT "E" coat, just a transport primer. The first part of the EDP process is phosphate coating of the bare steel. In the first step – pretreatment – metal is cleaned and a phosphate is applied to prepare the surface for application of the e-coat. This process is essential to achieving the performance requirements desired by product end users. PPG Coatings Services analyzes the metals to be processed and chooses the most appropriate chemicals for cleaning and phosphating. In our facilities, the most common pretreatment regimen for steel and iron parts is immersion in a high-quality zinc-phosphate system.
Hey, overspray; "I see lots of posts about putting filler over epoxy, but very seldom is the epoxy product brand/name mentioned or the cure time that is recommended and prep (sanding/scuffing) before the filler is applied. There are so many different "epoxy" primers, and they are not all the same." & "Which epoxy primers are completely water proof?" Appreciate the comments. So, what - or which - brand(s) of epoxy are the ones to use? IDK, so's I'm asking... While I'm not trying to get you to start steppin' on toes, I'm more interested in what works, the 1st time, correctly. Let's suppose, correctly, that I have some Evercoat plastic to use eventually. Which epoxies, by name, are compatible? BTW, I'm ~ 40 mi N of St.Paul. So similar to you in temps n climate. TIA. Seriously. Marcus...
For 27-years I have been indiscriminately putting 3M filler over whatever brand of epoxy primer that was at arms reach, on over 200 vehicles, and enumerable parts, and random objects. No callbacks. No complaints. No failures. I'm not saying that there is not a combination that won't work, but I have yet to find one. I'm not going to start worrying now.
Filler adheres to 120 grit sand scratches just fine so I’ve always sanded cured epoxy and e-coated metal before applying filler and working it out. It also helps with the high spots where you break through the epoxy because the epoxy doesn’t feather very well, at least some epoxy doesn’t.
Since I don't do filler over epoxy primer, here's some ways to test or at least discriminate some epoxy primers: the epoxy primer tech sheet will have instructions for application which usually includes: surface must be clean and free from corrosion--or--properly prepared, along with application temperature, flash time between coats, and recommended film build and cure time. Read the tech sheet: see what is recommended for recoat time without sanding, anything with an open recoat window of 3-14 days will probably absorb solvent and water. If the epoxy primer sands like it is "gummy", it is probably not cured completely and still contains solvent which could cause adhesion problems putting filler on top. Some tech sheets will give moisture absorbtion info, but very few do. Some manufacturers will list instructions for using filler over their product -ie- minimum film build, cure time and sanding recommendations before applying filler. The best test is to lay a wet rag on the epoxy primer (after the cure time) and see if it leaves a soaked in damp spot after several hours. Putting epoxy primer over cold metal can trap a fine layer of surface moisture under the primer the same as putting filler over cold metal. Some solvents in the primer may absorb the moisture, but some of it will probably stay in the primer layer. If you have an old car body you found in a pasture, most or all of the paint is gone leaving a beautiful rust layer, you need to prep the metal by removing and treating the rust before any primer or filler work. Epoxy primers do NOT chemically treat or neutralize rust. Epoxy primers adhere to the surface by mechanical adhesion NOT chemical adhesion. Even after sandblasting a rusty body, there is still rust in the sandblasted steel and should be treated chemically with metal prep/phosphoric acid. This system has been around for over 50 years and is probably the best for treating surface rust on metal when restoring cars. The epoxy he's using is very good but--expensive. The "E-coat" on the aftermarket parts is NOT E-coat, but only a "transport" primer, and almost all aftermarket sheet metal parts are cold rolled steel and have NOT been "phosphated" or have a phosphate coating on the steel like original (OEM) parts do. In the video he mentions not putting "self etch" primer over the "acid treated and phosphate coated" metal. After treating the metal (like in the video) the etch primer (which contains a small amount of acid) will not adhere well because the acid in the etch primer has nothing (rust/corrosion) to convert with.
https://www.aircraftspruce.com/catalog/cspages/metalprep.php Here's another REALLY good video from another "High End" restoration shop. Of course they are in California, where there is NO below zero weather. You see that they treat the metal before epoxy primer (NOT DP epoxy). They were using PPG VP2050, which is a very good high build epoxy, but very expensive for the DIY shop. The PPG DELFLEET epoxy is still expensive for DIY, but you decide how much to spend on your project. Notice the (PPG) temp and humidity gauge, when he is explaining body filler. And....as always-lots of hand sanding.
You are welcome. The main thing about a GOOD epoxy primer is that if you are applying over surface rust, that rust must be removed and neutralized. The best system is to acid remove the rust then clean/rinse and use the conversion coating to leave a zinc phosphate coating under the primer. This is how car manufacturers and OEM steel replacement parts are prepared. Rust will "creep" under most primers (including epoxy) without the phosphate coating. The "E-coat" (electro deposition primer) on the aftermarket parts is NOT E-coat, but only a "transport" primer, and almost all aftermarket sheet metal parts are cold rolled steel and have NOT been "phosphated" or have a phosphate coating on the steel like original (OEM) parts do. The truth be known, using the acid wash and conversion coating to properly remove and treat rust is a messy and time consuming job, and most shops or do-it-yourselfers don't take the time. When this is the case, "epoxy" primer is just covering the metal and there is no chemical interaction or protection.
If ya read the tech sheet, sometimes you’ll find it says “for best adhesion on bare steel use xxxx cleaner” not all of em recommend it
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