Google it. Many of the threads I saw discussed painting mufflers.
Who is talking about headers or exhaust manifolds? Christopher, the original poster, asked about:
I’m not partial to the look of a bare stainless steel exhaust underneath my XK120.
Since he said underneath I assumed he was wanting to paint his car’s mufflers.
Watched their video. Based on the rubber gloves and the water wash, I’m gonna guess that their chemistry involves hydrochloric acid. Do they have a MSDS for that stuff? Should say if has HCl in it.
Well, I have to admit that I had to Google selenious acid. Turns out that its primary use is for gun bluing.
I’m welding on plates of stainless 316 plates on my XJC where the rust traps are.
I’ve researched this quite a lot and the outcome of my research seems to be that coarse abrasion is the key ( See what I did there) to getting paint to adhere to stainless steel.
After abrading some2 pack epoxy primer/finish like epoxy mastic
https://www.rust.co.uk/product/cat/em-121-epoxy-rust-proofing-chassis-paint-7
I’m using lechler 29107 which is an ideal product for restoration bodywork but that has to be covered over with a top coat.
In some places where the stainless parts are visible I will try to solder coat first. I found a flux for stainless steel which seems to work.
So you’ve got to weigh up the work in abrading the exhaust, buying and applying the paint (and maybe bits of it flaking off where you haven’t abraded it enough) compared to accepting the look of the stainless steel.
For the headers you can talk to some ceramic coaters and see what they say, live with them, change them for the standard cast headers and send the stainless manifolds to me as I could force my self to live with them 
…onto surrounding steel?
Pray tell, how??
Use 316lsi mig wire and argoshield gas or tig if you are lucky and skilled enough to own a tig welder which I’m not!!
316lsi is the recommended wire for joining stainless to mild steel.
Aha!!! I might have to try that, sometime!
There is some discussion on whether a trimix gas is better, more on whether costs justify joining mild steel to stainless, which grade of stainless wire to use some say use the same grade of SS wire to your sheet, some say use 316l or lsi for all stainless steel to dissimilar steel, whether the joint not being corrosion resistant makes it not worthwhile. etc but put it into perspective, it’s just an old car, no fancy metals or high stress engineering but lots of rust traps.
I went for the 316lsi and use 316 SS plate of 1.2mm or 1.4mm for the rust traps and keep the joints out of the rust trap area and 2 pack epoxy prime everywhere. I used two 0.5kg rolls then decided to buy a 5kg roll and was struggling to get decent neatish joints. I eventually spotted that the wire that had been supplied to me was flux cored! I then brought, after getting my money refunded, some normal 316lsi 0.8mm wire and redid some of the joints.
It’s a bit of a hassle to change the rolls when I want to weld mild steel to mild steel.
Next time Im at my local friendly welding supply store–soon, since my argon bottle is almost empty–I think I’ll get a small spool of that, and give it a whirl.
Are you tig or mig welding
Tig would need argon from what I have read.
Mig needs a mixture argon plus CO2 or possibly plus CO2 and O2, I just use my 5% as for the mild steel and it seems to work fine. Again mixed messages on the internet.
MIG: I use 75/25, argon/CO2.
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You may need to get less than 5% CO2 or less and maybe 2% O2.
I use 95/05
That works for mild steel as well.
I’ve had good results using that mixture for the stuff I’m MIG welding. But I’m a neophyte at it, there may be better mixtures out there.
In general you can weld a surprising number of dissimilar metals. Lots of copper pot stills have steel reinforcing pads and steel legs welded to them.
If you are going to abrade stainless, you need to ensure that you never use anything that has been used on carbon steel. Abrasive blast is the preferred method of pre paint surface prep. Stainless is corrosion resistant because it has an extremely hard oxide layer. You intentionally remove that layer when you abrade. If you also introduce any kind of iron contamination, it will rapidly show rust stains.
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