Okay, so I picked up a 69 SS for a pretty good price and started working on it.
Through the years many holes have been drilled in the fire wall and floors. Both are rust free except for surface rust and was wondering the best method for filling all those damn holes.
Some are drilled 3/16 to 1/4" big and some are just from sheet metal screws. Should I go crazy MIG welding them shut or is there a bullet proof filler I can use with a backing material?
Some of them are tough to get to and the welding poses other problems.

HELP...

My firewall was the same way. I welded them all shut. Pain in the butt.

I would weld all of them you possibly can. Filler may come out down the road. It might not be as critical since it's not on the outer body, but if it's at all possible I would weld them. It might be a pain right now, but it will be a bigger pain next time. No sense in doing it twice.;)

welding the holes would be the best way to do it. Having someone hold a peice of copper behind the hole while you weld will make it much easier.

welding the holes would be the best way to do it. Having someone hold a peice of copper behind the hole while you weld will make it much easier.

That method works well.

The problem with using filler is that you can trap moisture in there, causing rust. This is what I've been told, anyway.

I agree with the copper...I welded up some holes in my trunk that were put there to hold the exhaust on...it was quick and easy with the mig and the copper. I took a piece of soft tubing and beat it flat, held it against the hole from the backside and just a few short bursts from the mig and it was filled. I used the grinder to cut it down and you can't even tell there was ever any holes there. The copper trick is really nice.

I've used epoxy glue. Sticks like, well, glue, and it won't attract any moisture. You can sand the stuff.

Id weld it but id recommend JimM method too, i did it on my car to test it and it worked well.

JB Weld will last forever and never come off. Like Jim says, it sands easy enough as well.

JB Weld will last forever and never come off. Like Jim says, it sands easy enough as well.

A couple weeks ago I would have scoffed at using JB weld for this. But after just fixing my spoiler with it, I have to admit it's cool stuff. It files and sands well. Holds well too. I'd still mig what I could get at, but JB Weld might be good for those you don't have good access too.

If you weld it using the copper backing, and don't have a buddy to hold the copper, a strong magnet or two to hold the copper on the back side of the hole works nicely.