Heat gun vs boiling water vs solvent soaks for a glued-on brass knob - what would you try first?

On the cars I've done this exact dance with seized threads a hundred times, and the combo everyone swears by is heat first, oil after. You heat the joint till it's good and hot, let it start cooling, then hit it with penetrating oil while it's still warm. The cooling metal actually pulls the oil down into the joint, which is why doing it in that order works and slapping oil on cold barely does anything. For your knob, a heat gun on low held back a few inches is safer for brass than trying to get it glowing. Brass conducts heat fast, so the whole knob gets warm quick and the glue shoud let go before you risk any discoloration. Boiling water genuinely does work on hot glue and some soft epoxies, I've freed stuff with a kettle pour, but if somebody used a proper threadlocker you're right that it won't be enugh.

One caution from experiance, go easy on torque when it finally starts to move. I snapped a brass fitting once by getting impatient right at the break-loose moment. If it fights you, more heat and another oil cycle beats pulling harder every time. And if it turns out to be threadlocker, some of that stuff specifically needs sustained heat to release, so a slow warm-up followed by patience beats one blast of the gun.

I'd push back on the heat advice for this one. Brass expands more than steel when it warms up, so if you hit that knob with a heat gun you can actually make it clamp down on the spindle even harder. I found that out the hard way on an old gate latch, heated the brass side, and the thing would not budge until it fully cooled again. The heat-first trick the other poster described is great for steel-on-steel fasteners, but a brass-on-steel joint behaves backwards.

My honest take is to not fight the joint at all. Spindles are cheap and easy to come by, so the knob is the only part worth protecting. Give it a long soak instead, a week or more in acetone with the knob sitting spindle-up so the solvent can creep down into the joint. Top it off every couple days. If it still refuses to move, cut or drill the spindle out. You can do that without much risk to the knob since the spindle is what you're sacrificing.

I saved a drawer pull from my own grandparents' place that way last year. The soak alone got it turning after about five days, no heat, no drama, and the finish stayed exactly as it was.

Another old trick that works with the thermal arguement instead of against it: stick the whole thing in the freezer overnight. Cold makes the brass shrink more than the steel, so the joint opens up instead of clamping down. My dad used that on seized bushings back in the sixties, before anyone had a heat gun in the garage. Worth trying before the glue route since it costs nothing. Pull it out, let the condensation dry off, then twist while it's still good and cold. If that fails, then I'd go with the warm soak and oil routine the others described.

Dumb question from me, but is there a way to tell what kind of glue it actually is before picking a method? Like does that red stuff look diferent from regular epoxy once it's dried? I only ask because I had a stuck part on an old faucet last summer and I just let it sit in a bowl of relly hot soapy water with dish soap overnight, then wiggled it a little each morning. Took like three days but it came free with zero heat and zero damage. Maybe too slow for you but it costs nothing to try while you decide on the fancier options.