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68 Mustang Coupe
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That's a Flux core weld. It will almost always be to hot for sheetmetal. Switch to solid wire and a 75/25 bottle.
 
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67 Fastback T5, 331 stroker, TCI Frt End, Canted 4 link rear, 3.55 gears
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Another consideration is the wire that you are using. I've never used flux core, but I know there is some cheap (solid) wire out there that produces less than desirable results.
There's a lot of splatter, which is usually from contamination, which in this case could be from the e coating.
 

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Top piece looks to be painted or coated in something as others have pointed out,…….
 

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Bare clean metal is a must, top, bottom, everywhere. Welding in, through or in close proximity to paint or other coatings will contaminate your welds.

Welders that don't have an infinitely adjustable wire feed make life tough for the newbie welder. You often have to set the feed at too high of a speed, and compensate by moving the torch faster. As others have stated, practice on scrap, not on your project. Go to a local auto body shop and ask if you can have some damaged sheetmetal parts and practice on those. Time spent here, will pay back ten-fold in the time you don't have to spend grinding your welds to make them look decent. It's the difference between being able to weld, or being good at grinding.
 

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When I try and plug weld, the puddle doesn't build up well. I'm afraid to hold too long, I don't want to blow through. I'm also afraid to turn the heat up for the same reason

This pic is me trying to fill the hole for almost 5 sec. I'm using the HF titanium easy flux 125 with .30 wire on its lowest heat setting, and medium low wire speed.

View attachment 855228
Tou can try clamping a thick piece of metal on the underside of the pieces you are welding to help with burn through. I have welded two pieces of metal together without drilling a hole in the top piece. Did this on the crossmember under the radiator support to the strut rod bracket. The result looks somewhat like a plug or spot weld.
 

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The pic looks like you are starting the weld in the middle of the hole. In optimum conditions with a nice welder that might be the best way to go (don’t know).
I am also limited to a flux core welder. I had good results by starting the weld pool at the perimeter of the hole and working the bead around, then filling the middle.
Yes. When you plug weld, instead of holding the wire in one spot in the middle, Move your puddle in a spiral starting by going around the entire perimeter and then spiraling in towards the center. Holding the puddle in the center will put a lot of heat in one spot making it more likely to burn through. It will also not produce enough heat around the perimeter to penetrate where it needs to penetrate the most. Also, with a flux core, the flux will get trapped around the perimeter weakening the weld as the puddle, left in the center, expands outwards.
 

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Discussion Starter · #27 · (Edited)
Thanks all. Looks like it was mainly a wirespeed issue. Just needed more wire in faster! Bumped wire speed from from 3 to 7-10 (depending on backing piece thickness) while leaving the heat at the lowest setting and cleared up the issue 90%. Working on technique to get the rest of the way there.
 
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