[Napa Skip]

I believe a magnet will be only marginally effective in trapping the fine reddish rust coming out of a Model A gas tank.

Short answer: While iron (and its alloys that are referred to as steel) is magnetic (generally speaking; an exception is most – but not all – stainless steels; try getting a magnet to adhere to your stainless steel headlamp shells or radiator cowl) during oxidation the unpaired d-shell electrons (which give iron atoms their magnetic moment) bond with – typically – oxygen with the result that the magnetic moment that results in ferromagnetic properties is lost. [I couldn’t find “…d-shell electrons…” in my copy of the Service Bulletins, but I’m certain it’s in there somewhere…]

Longer answer: There are numerous oxides of iron (about 18 or so), the most common of which are black iron(II) oxide (FeO or ferrous oxide); dark blue-black iron(II,III) oxide (Fe3O4 or magnetite) and reddish iron(III) oxide (Fe2O3 or – as a naturally occurring element - hematite). Usually, in the presence of oxygen and absence of a high-temperature acidic or basic environment, red rust is formed. In other environments, a tight blue-black compound such as Fe3O4 can be formed, the most common example of which is the bluing used on firearms (although the actual color of the “bluing” can be controlled by changing the pH of the bluing bath). Thus that midnight-blued rifle you’re so proud of is really just a rusted chunk of steel, albeit a nicely rusted chunk of steel.

Empirically, if you have a glass sediment bowl with red rust in the bottom (and no internal filter held in place by a steel spring) you might test this by attempting to drag some of the fine red rust up the side of the glass bowl with a magnet. You may get a small trace of sediment to drag along, but I suspect this is impurities in the rust. At least the fine red rust in the glass container in which I keep the rust that has been settling out in my sediment bowl for 35+ years does not respond to a magnet.

Filter comment: the unaided firewall sediment bowl (in my experience) ought to entrap most of this sediment, with a finger filter that fits inside the gas tank shut off valve taking care of the larger chunks. I’ve used a micro filter inside the sediment bowl but – in my experience and as pointed out in other posts above – they rapidly clog, leading to the bewildering experience of satisfactory idle and low/moderate speed operation but – at about 45 mph – leaning, loss of power and backfiring as the amount of fuel delivered to the carburetor falls below the amount needed to sustain high speed operation. Inevitably this occurs on a warm day leading to the ‘ah-ha’ moment of “by Jove, it must be vapor lock (or a failed condenser, or…)” only to discover that not to be the case.