If you do weld and plan on finishing out the weld I would TIG. I had three holes in my hull one was about three inches of a tear that I tried to MIG but every time I sanded down to finish, the weld was compromised (pitting) at the start of the weld where I needed it perfect but as the weld was built up (the last part) it sanded fine with no holes or pits in the weld. I don't have a TIG yet so I had to load it up and take it to have it TIG welded after trying to MIG three times with the same result. Here is a thread documenting my efforts about 6 years ago. Good Luck ..
My DR came to me with a patched gash, which is probably a good illustration of how NOT to do the repair. This cut was probably not as bad as yours. I must admit that I have not yet done anything to fix this. The patch is functional, if not pretty, and stopped the cut from getting worse. I've just been enjoying riding around in the boat for the 5 years I've had it.
Locomotion has provided a nice illustration photo sequence in a restoration thread on this site, showing how to do a proper flush patch. One of these days (years), I hope to do that on mine.
You can see the cut on my boat beside the rear seat in this first photo:
A previous owner pop-riveted a patch on the inside, to stop the tear from spreading:
And then filled the outside with something (JB Weld?) to try to smooth it over, which didn't work very well:
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Great info. Once I get the engine out, and on the rotisserie, ill get some better shots of the bottom. And yes, divide and conquer. Ive been warned about welding. Ive got a guy who repairs a lot of my outboard castings and has a laser welder for thin aluminum. I might quiz them on that technique and have them come to the shop and inspect the boat, maybe test some spots. I sure appreciate this info!
My DR had a lot of welding done on it by a previous owner. This included filling in a large hole in the rear dash from a helm. Most of the small bolt holes welds are gloppy on backside and ground smooth on front. The 3 inch helm hole has sheet aluminum backing it. You can tell a color difference between factory metal and repair work but its hard to see. My friend who was a 35 year welder at the GM plant said he would have backed them all, including the small bolt holes with small discs of aluminum. If the inside wont show, or you can live with the patch showing, backing it and having a quality welder do it is an option as well. Welding aluminum is tough I'm told, apparently cleanliness is key, extra clean metal to start with is essential.