I plan on patching the bottom of my fenders this weekend. I ordered patch panels from Year One. Anyone have any tips before I start. This is my first attempt at a fender patch. I have a buddy coming over with his mig to help.

There's a great how to article in Street Rodder magazine this month (August I think. It's on the newstand now) on panel replacment and hammer welding. Many of the techniques will apply to your situation.
Main things:
Make sure the panel is perfectly fitted before welding.
Tack weld in place and keep the panel cool.
Use short welds to keep the panel cool.
Have a wet rag with lots of cool water.
Take your time!

One question. Are your existing fenders totally rusted through on the bottoms and edges or do you just have a hole on the outer lower panel?
I would only use the entire patch panel if you have rotted out edges. If it's just a hole and the bottom and outside egedes are OK, just patch the hole with steel and keep as much of the original panel as possible.

Neither fender has damage to the edge. Also, both braces are ok. I planned on cutting the rusted parts out and then cutting from the replacement patch panels to fill in. Now that I think about it I may go and buy a sheet of metal and patch with that and send the patch panels back. Any thoughts!

That's what I would do. Save yourself some $$. Also, if you can get an older (pre-1980)unusable fender or hood, use it for the good steel. The stuff you usually find in a hardware store is crappy Chinese stuff.

I have read alot about welding here. Butt welding versus lap welding. Would a butt weld be used for this.

Yes the butt weld would be the weld of choice there. A butt weld should be used all the time when doing any type of resto work, especially on a spot like the fender. You just cant get paint inbetween the two pieces of metal and rust will form again.

If you take your time and only weld a 1/4 inch or less at a time you will done fine. Let the weld cool naturally you dont need a wet rag or air gun to cool the weld. If you use a wet rag you can shock the metal leaving it work hardned, meaning it will be very hard to hammer and dolly out any high or low spots.

Make your gap perfect, try to get the gap width of the wire that you use for the welder, I use .023.

Also before I weld together the tack welds I take my grinder and grind down the weld slightly. It helps the two welds become one elmilating pinhloes in the weld...Eric

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1970 chevelle
1970 chevelle SS455 not a typo its a buick baby
1949 and 1972 chevy trucks
http://briefcase.yahoo.com/bc/sevt_chevelles

Thanks for the tips. One question, what guage metal will match up well with my fenders? I am going to try the junk yard first for a old fender or hood but if I can't find what I need I will need to buy metal.

something like 18 ga will do fine. If you cant find any old hoods or fenders go to your local body shop and get some scrap parts that they have. We have to drive about 30 minutes to get rid of your scrap metal, so I sure that if you ask for some steel they would give it to you. But only get metal from a GM or Ford car. They seem to have thicker metal then the others. Also get it from say like a hood or boxside something larger and flat. Boxside have thicker metal then a fender does.

Its not that big of a deal to get the same gauge, something close will work. Keep in mind when welding just try and keep the heat on the thicker gauge of metal, point the nozzle at the thicker metal. Work slow if it takes three hours to weld that up so what...Eric

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1970 chevelle
1970 chevelle SS455 not a typo its a buick baby
1949 and 1972 chevy trucks
http://briefcase.yahoo.com/bc/sevt_chevelles