Sprayed my cab in one thin coat of Omni Epoxy Primer last August...had to store the truck under a barn for a year due to selling and building a new house. The barn was not completely enclosed. After pulling the truck back out I could find some areas where it looked as if flash rust was coming through the primer. I sanded all the noticeable areas as best as possible and scuffed the remainder of the cab and resprayed with a new thicker coat of Omni Epoxy Primer. I still have more body work to do I am now just worried about having some rust hidden under the areas of epoxy that may come back. I really don't want to go through blasting the whole entire cab back down to metal again. Do you think I am ok? The metal was clean with no rust before the first coat of Epoxy.

 

Bump...I know the right person has not come along to answer.

 

Did you originally take it down to bare metal or just scuff the original paint before you primered it last August?

 

Sounds as though he did take it down to bare metal which is what I have done with my entire front end. Yet after taking it down all the way, cleaned with soap and water, sanded with 180, I then washed it in PPG's Metal Cleaner and Metal Conditioner and dealt with filler issues before spraying the epoxy primer.

 

Oops, I missed that last night. (the bare metal thing).

Mike is correct, you have to put some type of metal treatment or etching primer on bare metal to help keep the rust from coming back. I probably did an overkill on the 41 when I sandblasted it. Ospho'd it, then self-etching primer, then the epoxy primer.

 

What it comes down to ,,is how clean you had it before the epoxy ..I don't put much faith into anything that doesn't etch the metal .If there was any microscopic rust ,epoxy won't help

Clean & rust free is hard to accomplish...

 

I am pretty sure it was as clean as possible. I sandblasted and then cleaned and cleaned with ppg dx330 wax and grease remover before spraying the first coat of primer. I think I am ok...just asking opinions, as I really don't think I am going to back up that far and take all of this primer back off.

 

If the metal was really completely rust free, and sanded or blasted, then no etch. metal conditioner or ospho like product should have been needed at all, a good epoxy primer should have protected it just fine. Some epoxys may also not be compatable with them. And as far as I am aware, metal conditioners are not recomended on blasted surfaces.
But I am thinking either rust was not really removed before, the metal was touched and not cleaned, or allowed to sit too long before covering (blasted steel don't take long to start flash rusting), or it didn't cure properly, ie mixed wrong, bad activator, applied in cold temps.
But most likely from him mentioning only one thin coat was applied, there just was not enough film build for proper protection.

Here are some lines from ppg's dp epoxy p-sheet
If repair areas show visual rust, we recommend that the rust be removed and
then use the appropriate PPG metal cleaning and conditioning system.
•
If bare metal has been recently exposed by sanding and shows no visible rust,
metal treatments may be used for optimum performance, but are not required.
NOTE: DPLF must NOT be applied over DPX170 or DPX171.(this is a wash primer and a self etching primer)
+ Film build of 1.2–1.5 mils of DPLF is required or the surface must be treated with
Metal Cleaner/Conditioner or coated with DX 1791/1792 (this is also an etch primer).

This is why reading product data sheets for whatever you use is so important.
Now epoxy if applied and mixed properly and enough build thickness, in theory it should prevent air and moisture from reaching below-Much like how a product like por 15 works, and with the epoxy preventing oxygen and moisture some things rust needs to form and spread, progress should stop, but if it is underneath, it will just be waiting to if given the chance. I sure wouldn't feel too comfortable about blowing on pricey paint, If I already have seen flash rust coming back through, about not taking things back down.

 

I think you are ok. When you sandblasted the metal it became porous and that why you use epoxy to seal it but sometimes when you are spraying such a large area you have some thin spots that allow a little air to reach the metal that starts the rust process to start over. Good news is that you can see the problem areas and sounds like you've taken care of it.
I had a sandblasting business and would put heavy wet coats of epoxy on and tell the customer to sand all over when they were ready to start body work, I learned this from the very reason that you are experiencing.

 

I think you are dead on Torkum...I was a bit rushed due to the fact I sold my house right before spraying....and I was getting low on material and was trying to make it stretch. I think I just thinned out some areas too much. I sanded all those areas back down to bare metal, scuffed the rest of the panels, cleaned with dx330 and then shot a heavier coat of epoxy. I am going to take my chances, hopefully it will work out. If not I will continue my learning curve :-)