This reminds me of when the young guys in my family tell Ancient Grampa how easy, really easy, it is to set up his cell phone. Grampa does not find it so
Hence, I have some perverted sympathy for you when I say it is easy to solder.
First, the strength of a soldered joint is very poor in tension, it is a shearing load that can be held by a soldered joint. You might "spot weld" but you ainagonna spot solder successfully.
THE STRENGTH OF THAT JOINT DEPENDS UPON THE SOLDER LAYER BEING VERY THIN. Solder itself is weak, the joint can nevertheless be very strong it it is thin. ("why" is a complicated mechanical metallurgy thing, just live with it)
It is quite beyond this Grampa's imagination what it is you are trying to do anyway "...tang spot soldered onto my bolster..." Tang? Bolster? Huh? "Spot solder"?
Anyway sand the thing bright - no need to file unless you wish to change the shape. Hold the two pieces together - first having smeared on whatever flux you are using. Heat it until flux melts or bubbles a bit. Then just touch the solder wire to it. You should get the metal just hot enuff that the solder melts and gets drawn into the joint by capillary action.
Try this on some scrap first, until these words make sense.
I happen to like the lead-free solder which is 98% tin 2% silver (98Sn 2Ag) The hardware I go to likes to dumb things down & not tell you what is in the solder, just Sales/Marketing gibberish. Anyway lots of them work, I just like tin-2%silver for what I do.
It might be nice to know what kind of solder & what flux you were trying.