Iron Manhole Cover

I may be dense but shouldn't a strong acid deteriorate (rust) metal? Cleaning the cover once with acid will certainly bring out the grains. Between pressings, I think you will do better using the correct solvent for the ink residue left from the previous pressing.

This is a city manhole cover, it weighs a lot and is really thick iron

So it's a stolen manhole cover, eh?

Put it back and make or buy your own tee shirt making supplies.

Hot or cold press? Ink type?

Cold press, printing it on Tshirts

POR15 works great in black or clear, spray or brush-on...

...or I guess you could have it powder coated...

I'm using a silkscreen ink, pressing Tshirts on it (hypothetically speaking)

So whatever solvent works on this silkscreen ink, use that to clean the typeface of this manhole cover.

Don't use anything in a spray can.

Use a two part epoxy that will chemically cross-link to form a much more resistant coating on the iron cover.

Many companies make such coatings and they are your best bet. They can be applied using any simple spray rig or even Preval - Refillable spray system - Spray any paint !

Do you use water-base or oil-base ink? If it's like paint, water-base will cause iron to rust. If you're not already doing so, try oil-base ink.

https://welogoit.com/blog/2018/05/types-of-ink-used-in-screen-printing

Then maybe a phosphoric acid wash primer, before epoxy coating?

Definitely something to consider. Phosphate etching usually gives a good tooth to bind to as well.

I see that I repeated myself.

Old-timers disease?

If you want to preserve the rough texture, you could mix up a saturated solution of copper sulfate in water. Painting on or immersing the cover in the saturated solution of copper sulfate the copper should trade places (plate out) from the iron. They used to sell copper sulfate crystals as a root killer for septic lines.

Ok clean with Muriatic then rinse well with baking soda and water maybe distilled water , then dry it quickly and spray it with something that is compatible with your ink like clear coat, clear lacquer, maybe even a metallic paint such as aluminum or chrome paint. You can check with most coating companies but get the rust off and neutralize it with the alkaline soda.

I've had good luck with ordinary spar or marine varnish for preserving the patina of iron objects I don't want to paint. First wire brush or otherwise clean, then I just slop the varnish on with a paint brush, rag, or whatever. Let it penetrate a minute or so, then wipe the varnish off before it drys; paper towels have done fine for wiping off. Then let it dry a couple of days. Even plain motor oil would do it, but it remains messy.

Maybe work around the problem rather than beat the problem. I don't know how many of these T-shirts you are (hypothetically speaking) printing and I don't know if it actually is important that you use the man hole cover. But, consider making a (negative) mold of the man hole cover in plaster, silicon rubber or some synthetic molding compound. Then, use silicon rubber, synthetic mold compound, poured aluminum or whatever to make a positive that will not rust. You can now make man hole cover prints without using the rusty man hole cover.

Are you actually using the full man hole cover? That is a rather big shirt. If you are only using part of it then this idea might yield a smaller and easier to handle printing device.

I am using the entire manhole cover, it is sitting in my yard and I have to way to cover it so I'd like to protect it against the elements as well as stop the rust from getting on the Tshirts.

Out the front door and off to work?

Morning dew much?

Is this manhole cover still covering a manhole?

No! I promise it isn't.

Color the t shirts with natural dye and use rust to print the image. It's a mordant.

Long term answer... take a cast of it to produce a female impression and then make a rubber male from that which will probably be better for taking prints.
Del