I have a worn part that I want to add metal to so I can refile and square it off . What kind of welder can be used and type of rod? I have an arc welder and wire feed . Thanks.

What is the part? what does it fit?

For just about any welding done on a firearm your best bet will always be TIG. Primarily because the heat is so concentrated in a small area.
JMO

Can you tell me if that'd work to weld shut the goofy hole in the receiver used to accommodate the hammer-blocking safety on the Marlin lever-action rifles? A few years ago I asked various gunsmiths if they'd do that and, surprise, no one would touch it! I've always been nervous about doing that anyway for fear of screwing up the heat-treating in the area of the locking lugs. Just wondering.

I think it could be done with a TIG welder and a good operator with no ill effects on the heat treatment of the rest of the action. If heat sink paste were used it would be even more of a sure thing.

BUT, gunsmiths have serious and legitimate concerns when it comes to REMOVING a factory safety device.

Our society is much too litigious for them to be comfortable with doing that.

I,too, feel the push button safeties on the later Marlins detract from the lever gun looks. They are really something that takes a lot of getting used to if you've used LA for years. I collect Marlins and Winnies so have some of the cross-bolt safety guns around. Being an old gunsmith I won't permanently disable the safety but instead put a rubber o ring on the red side which prevents it accidentally being put on safety. The trade off between the microgroove and Ballard rifling causes me to buy the newer or pre-1950 guns for hunting with cast bullets.
Riflesmith

Should have mentioned this too, if you weld up the holes, turn a piece of round stock till it fills the hole leaving just a slight recess. Weld around the plug and you need not worry about annealing the metal much. Also if the plug is fit just right on the inside you shouldn't have to do any work to ensure clearance of internal parts.
riflesmith

how about threading a rod in there and holding it with black lock-tite. Surface grind it and refinsh. I would try to order a new hammer to put it back to factory if you sell it.

Excellent advice from ancientriflesmith.

Welcome to the forum and keep on posting sir.

Oh yeah, I absolutely understand the reasoning! Thanks for the replies.

For the sake of conversation, consider fitting a brass round in the hole.