I've seen small (1 quart size) plastic bottles of muriatic acid sold in home improvement stores. In the section where masons supplys are sold,,cement, morter, trowels etc. Used for cleaning bricks and other surfaces.
Also if a friend or neighbor has a swimming pool,,they most likely have some on hand. Used for cleaning and adj. the water Ph so I'm told.
I ended up with a gallon that way when a neighbors above ground pool got tossed,,but maybe you can trade a beer for a small bottle of muriatic..
I usually just hit a zinc or cad plated screw with the propane torch and the plating burns right off in a few seconds. A quick wire wheel brushing and it's ready to go.
Acid or torch,,stay away from the fumes.
I just 'snug' them up to tighten them. Repeatedly removing and then reinstalling once perfectly timed screws will lead to them not being perfectly timed anymore.
One way to somewhat avoid the problem is to overtighten the initial fit. The repeated tightenings allow for still snug fitting with timed slots.
Most Best quality Brit shotguns have shouldered tang screws that leave the head timed perfectly. But part of the reason for the shouldered design is to make sure the spacing of the upper & lower tangs remains constant to avoid trigger and safety problems. When wood shrinks or compresses on these, shims are often found under the tangs to tighten the assembly up.