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Bond metal to metal
If the steel is new it likely has residual chemicals on it from the manufacturing processes.

I do know I am talking about when it comes cleaning regarding surface prep. I spent a lot of years at Boeing where we got lessons from the pros on surface prep before pai ting, sealing or adhesive bonding materials together. We had to go through certification classes for all those types of work. Acetone leaves the least residue when doing surface prep, alcohol does leave residue that can interfere with adhesive. Of course this is for doing bare metal prep. Not everything is compatible with acetone,
I've been thickening my surfboard epoxy with milled glass fibers, and wood flour, and the last time as I did not have enough wood flour on hand, sawdust.

For absolute maximum adhesion to steel, usually I have had rusty steel which I treat several times with Ospho. The rust turns black and hard and the paint primer over the rust bubbles and then comes off, but the good paint does not. I usually scrape off all the paint the Ospho lifted, then the bigger rust patches, now black, can be chiseled off revealing more rust below. One can keep applying ospho, wait for it to turn black, then chisel it off, all the way down to bare pitted steel if they really want to. Awesome product. Ace hardware has it for half price of any online purveyor.

The Ospho etches the metal. Scrubbing bare steel or rusted steel with a scrubbie pad and ospho, and good gloves, is almost cathartic.

If left on overnight there is usually a bunch of white flakey leftovers, which come off with acetone and a rag. Touch that acetone wiped Ospho etched steel, and one can feel just how well paint/ epoxy/primer, whatever, will stick to that surface.

Depending on the steel, it is not always easy to gouge some mechanical tooth into it.

The diamond coated cut off wheels on a dremel, can really rough up the surface nicely if one chooses the right rpm and moves the tool over the surface at a good steady rate.
Sometimes i will use a new sharp razor blade, and more than one new one as the edges dull, and gouge an X pattern into the steel, holding the razor at 60 degrees one way and then the other way to, then do the other portion of the X as well.

Sandpaper might work well too, otr it might not. It should be 60 grit or rougher, and once the small section of sandpaper dulls, say the portion under a fingertip, or equally sized tool, then move the tool face to a new section of sandpaper press as hard as one can, and pull sandpaper and tool across the area needing roughing. Moce tool to a new section of sandpaper, and repeat.

If one imagines the stress on the joint, then one can tailor the gouges in order to have the maximum possible grip in the orientation needed.

Here is a good chapter of System3's 'The Epoxy Book" which is very applicable to this topic:

Somewhere in one of the other chapters of the epoxy book,

....they say a lot of acetone is reclaimed with impurities which can affect adhesion. I've not had this issue, with acetone, but have with denatured alcohol and Isopropyl alcohol. If one wipes a section of the wiped portion, with distilled water, and has a good lighting, one can see if the water is repelled in places, as if there is oil still on the surface. Not so easy to see when one has gouged the steel deeply for maximum mechanical tooth, but it behooves one to actually completely degrease the surface previous to gouging that mechanical tooth, as if there are contaminants they can simply get pushed deep into the grooves where wiping will do nothing in their removal.

JB weld is awesome stuff. I think it should be in every emergency kit with baling wire and Gaffer's tape.

The bottom tank of my aluminum radiator was weeping where the tubes meet the tank. I could not really clean it as well as hoped for, but did what i could with what i had, thinned the JB weld with 91% isopropyl alcohol ( should really use Xylene for this) and sucked it up in a syringe and injected it at the bases of the weeping tubes.

5200 miles later, 2 cross country journeys, I've not lost a drop of coolant.
Yeeee diddly ding dang haw!
Yes, it seems the opinion is for JB Weld.

I took a wire brush to 2 samples of metal yesterday till they were bare metal.....and glued them together.

I’ll check on this Experiment tomorrow

1989 Honeywell motorhome
Ford E350 chassis.  460 engine
Well, at least the paint is off, that's got to be better than trying to bond to painted steel to painted steel.

That failure, is user error, plain and simple.

Forming mechanical tooth via the efforts I do could be several degrees of overkill, and wasted time, unless halfassery forces one to start over when the bond fails.

Wire brushing does not really qualify as 'mechanical tooth' and the wirebrushes themselves can introduce oils that they used to keep the bristles from to sticking to each other during production, and from corroding before use.

Even if the wire brush uses steel bristles, is that steel alloy of the bristles harder than the steel one is wire brushing? If it is not, it can't scratch it. The bristle tips are only sharp for a short while, and even if they could scratch the steel, which would form a better bond the bottoms of VVVVVV, or UUUUU?

0.5 acetone to remove oils/waxes contaminants on top of paint.
1. Wire brush to remove paint,
2. Acetone to remove contaminants, again.
3 Sharp new 60 grit or rougher sandpaper and/or a razor blade to gouge X shape scratches in both mating surfaces.
4. mix parts A and B as precisely as possible and apply to both surfaces. Do not overclamp, do not stress joint until full cure has been achieved.

Etching the steel with Ospho can be or similar Phosphoric acid laden product can be used upto step 3, or after if step three is repeated afterwards.

Mixing parts A and B of JB weld by volume, Ideally would be an exact 1 to 1 ratio.

Judging the size of blobs squeezed from the tubes A and B, can lead to huge errors as they are slightly different viscosities, and the height of the blob squeezed, if not accounted for, can make one think blobs A and B are the same size, when blob A is 50% taller and has 25% more volume.

JB weld is an Epoxy.
Epoxy is not like Bondo, where the amount of catalyst used, affects only the cure time.

When the ratio of epoxy parts A and B are off, the adhesion and strength of the cured product is some fraction of what it could be, compared to if the ratios of A and B were indeed perfect.

My fiberglasss laminating epoxy, one has about 2 to 3% allowable error in mixing A and B, before the results are obviously inferior. It still cures, still sands, still bonds OK, but it is still slightly soft, and can be peeled off the substrate with far less effort than should be required.

Making lots of small batches of this epoxy, which is a 2:1 ratio by volume, or 100:45 ratio by weight, is a thousand times more accurate going by weight.

I still chide myself for thinking I did not need a digital scale, as when I finally got one, all epoxy batches came out perfectly, as I would get them to the hundredth of a gram, and each drop of hardener/ part B, is about 0.05 of a gram but that depends on teh temperature of the hardener. Before teh digital scale I used to convince myself I could mix batches of 7.5ML by volume accurately. Larger batches are easier to get the ratios right, but that wastes epoxy. So much cursing was caused by thinking I mixed 5ml and 2.5ml when the meniscus of both in the mixing cup could lead to 30% errors even if I got the levels perfectly in teh middle of the graduations.

And temperature is another consideration. JB weld being an epoxy is an exothermic reaction. large masses of epoxy generate a lot of heat. which can expand the substrates trying to be bonded. If one side is hot from sunlight and the other colder, during the cure, the adhesion will be compromised to some degree. If large mass of expoy is used both surfaces can be heated locally, and if different, could expand and contract at different enough rates during the cure to compromise the bond.

Ideally one holds the joint mechanically together while the adhesive cures and does not disturb it during that cure.

Achieving Ideal can be a bitch, but achieving failure, is likely more of a bitch.
Is this for high shear, impact or peel strength bonding or all the above? Factoring in any resistance to chemicals, temperatures and UV will make it easier to buy once and forget it. The right adhesive is available once you define your application. There are several manufacturers, if you haven't already I'd try the 3M website first.

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