Scuffing Epoxy Primer

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gerahead

gerahead

Glutton for Punishment
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I am using PPG Omni epoxy primer on my project. The tech sheet states that if it isn't top coated within 48 hours (I think) it has to be scuffed before you can top coat it. I have been priming as I go and virtually none of the car will meet the top coat time frame. My question is........ once the epoxy is scuffed, is it good until you top coat it, regardless of time? Or...does the same time frame apply once it has been scuffed? I am trying to plan out my attack and need to account for this critical part of the process. Thanks for the help! L8r

Jim
 
I am not a paint expert but I would say the reason for the scuffing is to remove impurities left on the surface. If that is the case then I think you are in the same boat 48 hours. I could be wrong but my logical guess.
 
I too am not a paint pro but I think the time limit has to do with binders - once it sets up over 24 or 48 hours it get surface hard and won't bond to your top coat . I use a epoxy primer last time also and once done with the priming it was a week later b4 I could shoot paint on it.But that was ok cuz I wet sanded the whole car (with 600) before I painted it and got the best finish yet . with no lifting or bubbles
 
The time window is for a wet on wet application, your first coat of primer paint or whatever gains its adhesion from mechanical grip (sand scratches) every coat after that gains adhesion due to chemical adhesion. Once the primer or paint dries it needs to have sand scratches back on it to gain mechanical adhesion. I don't suggest sanding epoxy primers including PPG or Omni epoxy's. They are NONE SANDING primers. They sand like bubble gum and clog paper and scuff pads. You should spray your epoxy then cover with a sandable primer filler. The other reason you shouldn't scuff epoxy is that you cannot gain proper sand scratching for the filler primer to adhere. A grey scuff pad (scotchbrite) is only 600-800 grit, and a red is about 320-400... far too fine for proper filler primer. I would suggest 180 then filler primer, possibly 220.

Another way to eliminate this is to just use a corrosion resistant filler primer like NCP-270. It has great corrosion resistant capabilities and has excellent filling. The only reason people suggest you use epoxy is it is great for corrosion resistance and has good adhesion qualities for almost any surface.
 
And on a side note, with filler primers you want to sand and scuff them very closely to the time you top coat them, the sand scratches can actually shrink and close after sitting for a long amount of time, then you loose your mechanical adhesion again.
 
I have been in the bodyshop industry for the last 10 years or so. I have sprayed only PPG products. What i would recomment doing ,if your epoxy primer has been sitting over 48 hours, is scuffing it and coating it with another coat of epoxy, then hit it with your primer surfacer within 48 hours. This way you will be guaranteed to not have any bare metal spots under your primer surfacer and you will not need to use any etch primer.
 
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