Where did I go wrong? Rust showing up on Epoxy.

[jeepermat]

I sandblasted, and epoxy primed a rear axle back in August along with a car body. Blast material was "Black Beauty".
After sandblasting I cleaned the axle with waterborne grease and wax remover, then shot the epoxy after ~1hr, in total I did 3 coats to the axle, and car at the same time.
The axle has sat inside on my shop floor since August, I picked it up to start assembly and noticed what looks to be rust coming through parts of the epoxy on the cast center section.
The car body had also been sandblasted at the same time, with the same material and does not have any of this coming through.


Any idea where I failed?
How would you fix it?

 

[Bob Hollinshead]

Trapped moisture most likely, I've also seen it happen on cast surfaces-rough surfaces where there wasn't enough time between coats of epoxy-it runs off the peaks and settles in the valleys. The axle tubes are fine though right?

 

[jeepermat]

Trapped moisture most likely, I've also seen it happen on cast surfaces-rough surfaces where there wasn't enough time between coats of epoxy-it runs off the peaks and settles in the valleys. The axle tubes are fine though right?

Axle tubes look fine, only on the cast portion.
I waited 24 hrs between the 2nd and 3rd coat, I would think it should have been dry.

- - - Updated - - -

Trapped moisture most likely, I've also seen it happen on cast surfaces-rough surfaces where there wasn't enough time between coats of epoxy-it runs off the peaks and settles in the valleys. The axle tubes are fine though right?

Axle tubes look fine, only on the cast portion.
I waited 24 hrs between the 2nd and 3rd coat, I would think it should have been dry.

 

[Barry]

Like Bob said and only other thing is epoxy was not thick enough in those areas, we see this happen edges of frames where overspray make it look covered and it is not.

 

[strum456]

I have seen where non-spi products have done this, due to the "peaks" of a coarse blast profile not being covered with a relatively thin product.

I am interested in learning about water contamination. What did you mean by that, Bob? Do you mean water in the pores of the cast iron?

Could it have been waterborne wax and grease remover in the cast iron that was not given enough time to evaporate? I always worry about that with anything that is porous. I usually blow everything with an air nozzle after w&g remover to make sure it isn't laying in any nooks and crannies. On porous parts, I use a heat gun too. This is in addition to letting the parts sit 30 minutes or so.

 

[crashtech]

Just wanted to throw out that it doesn't look like three coats to me, not like I spray them, anyway.