The Stovebolt Site Forums The Shop Area Paint and Body Shop Here's what I don't get

If I apply epoxy primer over bare metal, why do I have to apply primer sealer over that? The epoxy IS a primer and what am I sealing? the epoxy? why? Do I have to seal the primer sealer? Then primer surfacer? and seal that? By the time you put on all the stuff they advise, you don't need paint!!! Then the paint gets sealed with clear.I live in AZ, there is no humidity except in the shower. Submarines don't have that much protection.

If you can wet sand the part without any breakthroughs or the need to add any filler, then yes, you can consider that application your "primer sealer" and spray the base over it.

I've read that you can sand, fill, repair over a coat of epoxy and then apply another coat of epoxy primer, especially if you break thru. What is the goal of putting something on epoxy primer? To smooth it? Give a better product to sand? To get adhesion? It's not to "seal". I think seal is misleading? Don't the coats of paint fill a little and smooth out, provided the epoxy is smooth? The only thing I can determine is that epoxy is a bear to sand. Gums up, etc. So the other stuff is what you are "smoothing". Don't see any other reason. Over the years I've read so many how-to's on painting that I'm over loaded. Every shop, every blogger, every company has a different approach. They stress "the prep is all important" then they all disagree on what that is. House paint (which is outside all day, all night, all year) lasts for 10-15 years. That's all I want on a Truck.
Signed, Earl Scheib.

epoxy may need a primer surfacer to cover imperfections, but it doesn't need a sealer - the "important" prep is that the epoxy or other primer is smooth, if the epoxy is smooth enough to satisfy you for top coating [even with house paint ], nothing else is needed

Bill

That makes cents sense. Thanks.

Some epoxies sand better than others. SPI and House of Kolor are some of the ones that are better at sanding with minimal if any loading of the paper. Now they don't sand as easily as a high build primer, but they are much better than most epoxies. I do know that H/K has better build properties than most epoxies, they advertised their KP2-CF as filling 40 grit scratches with no shrinking back. Not that you need anything that coarse.. But for something that may take a few coats of epoxy to cover any imperfections, an epoxy with high build properties may be a better solution to eliminating shrinking in the scratch areas as it is a more stable substrate.

In the collision industry, it's a quick in and out the door in 3 days or less (in most cases) so epoxy is not on their list of go-to products. They are more in line with etch primers, high build, spray and go. The typical epoxy takes a good 24 hours or better to fully cure, so it definitely is not something a body shop would want to use. But for a home hobbyist or restoration shop, the better sealing properties of epoxy in a longer term repair process before final topcoating make the epoxy a better solution all around. After all the body work is complete, a sealer is used to both give a uniform color (especially under a translucent, like red) but also to provide a tie coat that does not have the porosity of the high builds or filler that may have been used beneath. Spray a basecoat over any of these porous coatings and those areas will act as a sponge, and any solvents may have adverse effect on their stability, possibly causing shrinking effect or die back in the finish down the road. Most epoxies can be thinned slightly (<10%) to be used as a sealer with good flowout.

Thank you Robert. your knowledge is appreciated. Does "shrinkage in the scratch areas" mean that the primer will follow the contour of the scratch and still look like a scratch because of shrinkage? How does primer "shrink"?

One of my previous co-workers was in San Diego, on travel for work.. He saw an early model Porshe for sale out there and fell in love with the car, so much that he flew back a couple weeks later, bought it and drove it back east. I never looked at it that closely when he first got it, and it always looked like a clean little car. But about 6 months later the metallic silver sitting in the sun started to show defects (they may have been there the whole time, sunlight does wonders to show defects in metallic). You could pick out every stone chip across the front of the hood, and every sanding scratch that had been "filled". This is typical of the results seen from using a non-catalyzed spot putty (the lacquer based type in the tube) as it will shrink over time, especially given exposure to solvents from subsequent paint coats. Wasn't long and the door bottoms were showing rust bubbles, and it was obvious he had been taken for a ride on a car that had some poor repairs. The clearcoat still felt smooth to the touch, but visually the silver metallic mica showed every chip and scratch in the car.

This is why is is so important to use catalyzed glazing putties, like the Evercoat 416, or using paint products with high fill and no shrink properties. For the metallic to show those defects indicates the shrinking occurred after the silver went on. Cheap materials, cheap results, wasted effort. To add to that, minimize the sanding scratches by using a reasonable grit paper. IMO there is no need to use anything coarser than a 60 grit.


Missed one of your questions, how does primer shrink? Some of the primers that are not catalyzed are in a state where they can "soften up" given exposure to solvents. Some topcoats may contain solvents that react with various primers (try some spray bomb primers, see how well the wrinkles show up).

Explained well. thanks.

Shrinkage issue...my take. Epoxies wont do it, lacquer based material shrinks initially. Before epoxies, painters used lacquer based primers with good results and bad. Yes, I had a car repainted with lacquer primer that eventually showed the sanding scratches in some areas, others no. Here is what I think is going on. Lacquer primer shrinks as the solvents evaporate. It takes time for the solvent to evaporate. But, once they are gone, the shrinkage stops. So, if you prime with lacquer, you MUST wait a period of at least one week in good evaporative/drying conditions. Do you blocking sanding AFTER this period and don't add more lacquer before sealer or base coat. yes, I have applied finish paint directly to lacquer primer without issues, but safer to seal first. But sealing will not solve the shrinkage problem of lacquer primers if you don't let the primer dry properly.

Thinking of painting your own vehicle to save money, learn a new thing and to say I did it, is on my mind. Then I see how scary it is. Then I think how scary it is to pay big bucks to someone who may or may not screw it up. Even if they did not screw up another guys vehicle. So many solvent issues, compatibility issues, primer sealer issues, safety issues, temperature issues, dust issues, paint gun issues, drying issues, single stage, 2K, slow, medium, fast activator, reducers, induction time, flash time, enamel, epoxy, lacquer, urethane, water base....... media blast, chemical remover, soda, dipping, sanding.... I hate sanding. It's the one place in auto "restoration" where you can't say "you get what you pay for". You could possibly get a good paint job for $600 and a bad paint job for $6000. It all depends on the moon, the humidity and luck.

I swear by epoxy sealer because it takes me so long to get to that stage. After blasting, DX330 to clean, then epoxy primer. But then due to all the dents, imperfections, etc, K36 high build primer and then Rage Gold (bondo) and then more K36. Once I have it down to 400 grit dry and everything is as it should be, epoxy sealer. Why? Because epoxy sealer heals 320 grit sandpaper. So I go with 400 to be safe. If you don't use sealer, then the proper way according to the restoration colleges all over the country is to use 600 wet, no sealer.

So, its just easier and more sensible NOT to use water EVER before the base/clear. So 400 dry and sealer is just prudent. I understand this better than most people because I had a friend who was a disciple of the colleges and used 600 wet on MY project. Things happened and two years later, I had to replace rusted out metal BECAUSE of the 600 wet! Now after learning all this, its not a question anymore, its an ANSWER! 400 dry, epoxy sealer after all the work to seal in the work, base/clear. Stick with PPG, or whatever your favorite is, but stay with the brand all the way thru, listen to people who do this for a living like on autobody101.com forums and it gives you hope. Which is a much needed commodity when you are doing all this yourself!