I have a steel but plate that will not yield up its screws to, so far, penetrating oil, heat, and taps with a small hammer.  Any suggestions for what to try next?  I want to clean up an 1873 Trapdoor without using abrasives; I only want to remove years of dirt, paint spatter and general scrapes and, what appears to be heat or even some small fire damage to some areas.  Most of the screws turn easily, but the rear sight and butt screws are firmly in place.

After I get screws out, the plan will be to soak the metal parts as has been discussed on this site before.  Wood plans are still in the future and after I can remove the metal.