Omni acid etch proper use/prep before hand

I have a small project aluminum box that I am going to prime with PPG Omni products...the MP176 Acid Etch and the MP171 Epoxy primer.

I'm going to have plenty left over, and if all goes well I plan to keep using these primers in the future. I'd like to know more about using them.

The aluminum is being cleaned/scuffed and should be primed in a few days time, I think the sitting a bit shouldn't hurt it?

And onto my usual projects...cast iron or steel. I'm assuming that if I sandblast that the epoxy would be plenty enough protection for the clean metal. On lots of small bits I might end up using a wire wheel on a angle grinder to clean up the metal, removing all the rust that I can, though I know there still might be some in pores or small places...would the acid etch primer do well to stop this rust as long as it's covered with epoxy afterward?

I've always just been a bit confused if acid etch needs to be used on absolutely clean metal or if it's good to use on "slightly" rusty metal, for lack of a better term. I definitely don't mean flaky thick rust or something, just any small bits that might be missed. How does it react to any paint that might be left on a surface?

Thanks,
Brandon

I used acid primer on these wheel weights made from armorplate. In the 1970's I asked the paint shop where I worked on which primer to use and they said acid primer. They even gave me some since it couldn't be kept overnight. They said I had to use it when I got home. I never top coated them until 2010. They had very little rust. I sanded them and used epoxy primer
over the acid primer and used acrylic enamel after spraying them with a surfacer. Hal

Brandon, You question is one that is fairly hotly contested on this forum. I have had tractor body panels that I have had pros paint for me that after 3-6 years I can see little rust bubbles forming in places where there would have been pits prior to the refinishing. Just prior to the appearance of these bubbles on these panels, I was sandblasting and painting a dump truck body. When picking up the paint the counter man told me that I also needed acid wash. I had not heard of it so he explained how after you take the metal down to bare metal, there will always be some rust left at the bottom of the pits, so you need to "wash" with the acid prior to priming, if you don't want your new paint to bubble. He also advised wiping the surface down after the acid wash with a abundant application of a final prep product to remove moisture to prevent flash rust. There are a lot of phosphoric acid products on the market for converting rust to black iron oxide which is inert and is actually a barrier protective coating on steel and cast iron. The abundant final prep application also removes excess phosphoric acid residue. If the wipe down is inadequate, upon drying a little white residue will be left This residue can be removed with a little brushing, as with a plastic bristle scrub brush. The acid wash is even more important on cast iron than steel due to the pores in cast iron which will easily harbor rust molecules. CNKS will soon post his rebuttal, that epoxy primer will penetrate to the bottom of pits and pores, just as well as the acid will chemically "eat" it's way to the bottom.

Forget MP176, it is not a rust preventer or a rust converter, it is designed for body shops who use new metal and do not wish to sand. You will be sanding or wire wheeling everything (at least you should be), making 176 redundant. Just use MP170/1/2. Do NOT use 176on cast as there is a possiblility that it will lift the paint. If I am not able to remove the rust mechanically I will use Picklex 20 or Must For Rust which do not need to be washed off, they can be scuffed and have a wax and grease remover applied. I do not use anything containing acid on cast for fear it will lift the paint, as it can be trapped in the pores. If I remember right, Showcrops problems were not caused by him, but by a so called professional who hurried the job, did not remove the rust and caused problems. I am not incuding the B maniac and Glennster in that category as you can believe 99.9% of what they say (no one is perfect).

Showcrop, I use Picklex 20 or Must For Rust on pits on sheet metal. There is no good, safe way to get it out of the pores on cast. I did not say that epoxy will penetrate to the bottom of pores, it won't. I imagine that if there is rust activity in those pores that epoxy will at least slow the rust down. It is a good sealant, whether or not it will prevent rust from coming to the surface I don't know, as I have never seen rust lift anything on cast on my tractors. Rust is not my problem, it may be yours and others that live in humid climates, especially in salt air as you do. If anyone actually knows how to safely seal or remove rust from pores in cast I would like to know it. And, one person posted on this forum a few years back that claimed that etch lifted the paint on his tractor about a year later, true or not I don't know. I just don't think it's a good idea.