Wirewheel. The ones which fit on drills, are nice, when one cannot turn the tool around. Just put it in reverse and one can get off much more rust.
Ospho on a Sponge scrubbie. The 99 cent store ones are good enough, but scotchbrite is best, scrub until all metal and nearby paint has only slight amounts of rust color, wipe it up, and dab pm a final coat of green ospho, and walk away.
Ansell 88-394 AlphaTec gloves, or similar 20mil+ thick gloves, can handle such abuse.
When the Ospho soaked rust turns black, one can take a sharp cheap chisel, and lift up the bigger rust bubbles and where the rust had gotten under the nearby paint.
Chiselled Rust bubbles will be brown, Apply more Ospho, and scrub some more.
This can be repeated until there is just pitted etched steel, if one wants to take it that far.
Once Ospho is left on long enough to leave some white flaky residue( usually 12+ hours) , brush it off, then wipe with acetone on a rag until rag does not change color..
This Ospho'd surface will bond well with anything applied atop, bond very well, but mechanical tooth can further increase subsequent bonding by a large degree. Think sharp sandpaper in an X pattern, or even using a razor knife to scratch gouges into the steel Or even tiny shallow holes drilled at opposing angles.
I use epoxy, designed for saturating fiberglass. It has the viscosity of warm to hot honey. I apply a thick coat, then lay a prewetted portion of epoxy saturated fiberglass tape, or several layers depending on strength desired.
Epoxy has MUCH higher adhesion than commonly used/ easily available polyester 'fiberglass' resin, and is much more impervious to moisture/ oxygen intrusion than many layers of properly applied quality paint. One can apply a thick coat and have it cure to a thick stink free encapsulating film, in 4 hours depending on the exact epoxy and temperature
Epoxy is also more flexible and can expand and contract far better than polyester resin with temperature.
We've all see hideous fiberglassing, where polyester resin was used on a poorly prepped greasy painted rusty surface, to which it never bonded or bonded so poorly it popped off the first time the floor nearby was flexed with a heavy foot.
That, is not comparable to using Epoxy on properly prepped etched steel, especially when one adds significant mechanical tooth.
Epoxy, if it is to see UV light, needs to be painted.
Epoxy needs to be mixed to the prescribed ratios as exactly as possible. Do not adjust these ratios if it is a hot or cold day, as one would with Bondo or 'fiberglass resin'.
The fiberglass laminating epoxy that I use, has a 2:1 ratio by volume, or 100:45 by weight.
I use a digital scale as the graduations on many mixing cups can be way off, and if one misses by more than 2 to 3% then the cured product has some fraction of the strength, hardness, and adhesion strength as it would have, had it been mixed precisely.
My mixing stick is flat, the bottom edge fits the corner of the cup, and all 5 square sides get scraped on the same exact spot on the rim of the cup. This insures faster complete mixing of parts A and B.
The epoxy must be thoroughly mixed. Use good light and reading glasses until there are no visible swirlies, then scrape the sides of the mixing stick and cup one more time just to be sure.
If one cannot be bothered to be precise in the ratios of A to B, or can't be bothered to insure Epoxy resin parts A, and Hardener B, are thoroughly mixed, then don't even bother trying.
Paint the precut fiberglass patches, which are on some nice clean crease less cardboard, let the fiberglass soak it in rather than trying to force the resin to the weave.
While it soaks into the fiberglass, go paint the treated prepped steel.
Return to fiberglass and remove excess resin with a bondo squeegee.
Lift prewetted fiberglass patches from cardboard with edge of squeegee and lay in place.
Typically there is a bunch of extra epoxy as one mixes more than they need. There is alsolikely some extra time to apply this extra epoxy on something else, like some wood or something else which needs a strong glue, Good to have it already prepped for this extra
Epoxy slowly gets thicker as it cures, but if left in a large mass in the mixing cup will get hot quick and thicken quickly, and even start smoking if it isa a large mass on a hot day, so once thoroughly mixed, get it out of the cup, or spread it thin.
Epoxy does not bond to plastic baggies. One can lay such plastic over curing epoxy as it thickens and force excess epoxy from the fiberglass to teh edges, and mold it like clay. Push the edges flat, or have a bigger piece of foam rubber and a flat piece of wood and a weight to keep it in place as epoxy cures. If fiberglass turns white under these ministrations, you pressed too hard.
I've tried to chisel off fiberglass applied to steel with the procedure used above, and could not easily pry it off, much easier to sand it off.
I've yet to have any failures, and many are highly stressed areas.
Like most everything the result is all in the prepwork, not just going through the motions, but understanding the reason for those motions.
For example, one can use dull sandpaper and it looks perfectly scratched to teh naked eye, but sharp sandpaper leaves sharp V shaped valleys and Mountaintops, where as dull sandpaper has U shaped and will have a fraction of the surface area to bond to.
When Filleting corners is required, I will add milled glass fibers and some wood flour or other thixotropic agent to the mixed epoxy until it reaches peanut butter type consistency.
Achieving Secondary bonding, when epoxy cures in humid conditions, even with so called 'blush free epoxies' can require washing off the water soluble blush, and sanding some mechanical tooth before attempting to bond anything to it.
It's best to not have to employ secondary bonding, if one does not have to.
Relying on a chemical bond during the initial cure is superior, when possible.
I know that cutting out and welding in a new piece of properly formed steel is the right way to do it, and likely less work than the Epoxy saturated fiberglass method, and all the significant prepwork in volved in achieving a good to excellent bond.
I personally don't have a welder, but do have all these other supplies on Hand, and the skills to use them, properly, effectively, so that's what I do.
My Favorite Epoxies are those made By System3, but they are pricey.
Here's a link to some cheaper stuff I use for metal encapsulation/bonding.
Quickset clear Surfboard / Marine Epoxy Resin System works well in all fiberglass, carbon, and Kevlar reinforcements. Excellent in a surfboard, boat, and aerospace applications.
fiberglasssource.com
Fiberglass tape can make such tasks easier and cleaner, but sharp scissors are required either way.
Do not use fiberglass matt with epoxy, that material requires the styrenes in polyester or Vinylester resins to dissolve the binders holding it all together.