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I'm getting to the angle strips and bed side repair on a 1955.1 3600 and was wondering if the size of the wire matters for plug welds. Will I get better penetration plugs with thick or thin. It is a Lincoln flux core welder with .030 or .023 options on wires. Thanks, Brian

Correction: .030 or .035 wire options Lincoln flux core model 125

I don't think the wire size really matters. If you're using smaller diameter wire, you might have to up the wire feed a bit. The best thing to do is find some material to practice on (bed sides are 16 gauge) to get your welder set up.
Penetration isn't a real big issue with plug welds. You just need to be sure you don't blow thru the back steel, and get the edges of the hole welded all the way around.

Thanks

I use 030 in mine , but it will also use 035. A buddy of mine has the same machine and loves the 035.

Ive welded up to 5/16 mild steel, exhaust pipe ,1/16 and body metal on an S10 truck. All with 030 👍👍

My welder is a fluxcore Titanium 125.

Thinner wire has more of a tendency to hang up in the stinger and create problems unless the wire speed is carefully adjusted. Flux core wire is my least favorite- - - -it splatters sparks all over the place and the slag must be cleaned off the welds before priming or painting. I keep a small angle grinder with a stiff wire wheel on it handy for slag cleanup. Gas shielded bare wire makes a much cleaner weld that requires little if any cleanup before painting.
Jerry

I agree with Jerry on gas shielded wire being a better choice, but if you don't have a MIG machine, flux core is what you're stuck with. I've done all of my MIG welding with 0.023 solid wire, and switched to flux core only for miscellaneous welding I've done outside or on stuff like rotisserie brackets that I've fabbed out of angle iron. For that stuff, there's not a need to get the metal sparkly clean like is recommended for MIG. Haven't had significant issues with the smaller wire hanging up in the gun - just match the tip size with the wire used.

"Filling large grooves is easier with a higher WFS, but excessive WFS for the welded thickness can lead to burn-through."
Excess wire feed speed will not necessarily cause burn-thru. You can tell when you have excess wire feed speed when you start feeling the wire bump in to the metal you're welding. You're more likely to burn through if you don't have fast enough wire feed. Just my experience talking here.

My MIG machine has 4 power settings and variable wire feed speed. Each one needs to be adjusted for each specific welding job.

Kevin,
Thank you for taking the time and for the very detailed explanation. Great information. I am in the beginning stages of body restoration and practicing on donor panels. I learned real quick that to much heat is your enemy!!

Thanks again for the support.

Kevin nailed it! PRACTICE!!!!! Find some similar metal to what you're going to weld and dial in your settings for that session. If, no matter what you do, the .035 isn't working, switch over to the .030 and try to dial things in. Or vice versa. Dial it in on scrap until your happy with the weld and are in a place where you recognize and can adjust to mistakes quickly. Similar to painting. You don't just load up the gun and start laying it on. You do a quick test spray to dial in your pattern and atomization. I've welded with all kinds of machines and filler material. It all needs to be dialed in. Whether sheet metal or angle strips for beds.

the "too much heat" you speak of may also be a factor of not enough wire feed speed going into the puddle. If there's not enough filler for said amount of heat, its going to burn something, namely the parent metal. I find more heat, more feed speed, and LESS time on the trigger will get you a flatter weld that's easier to planish and clean up. And as Kevin said, practice on some scraps of the same thickness to get the machine AND yourself dialed in.

A professional welder told me how to weld plugs without blow thru that work very well. You use a non-ferrous metal on the back side, (aluminum, brass, copper) and it vastly lessens that chance of blow thru. The non-ferrous backing will not weld to the steel. A lot easier if you have a helper hold the non-ferrous backing in place. Use a pair of pliers or screwdriver to hold the backing metal as the heat does transfer.

Many of those backer tools can be found on the market. For a cheaper version, a smashed copper pipe can be formed into any shape you need. Make it out of long enough pipe, and it can be a one man operation with the holder far removed from the heat source.

Here's one backer tool that I have like Mark mentioned. It's magnetic (the black part) and can be used hands-free. Although I've never had the need to use it for plug welds, just butt welds and hole filling (like welding where I had removed a sheet metal screw holding two pieces together temporarily.)
I also have a copper bar about 1.5" w x 12" long that's handy for long butt welds, and a couple custom smashed copper pipe pieces. Attaching a handle to a piece of copper pipe is useful as well.