I have a couple left over rolls Silicone Bronze wire for my MIG.
Anyone use it for for auto sheet metal work? I heard that fillers don't adhere well.
I like to use it for lower temps and easier to grind.

I like it for sheetmetal for the same reasons. I think it works great though I've only tig welded with it.

It was the only stuff body guys used for over a half century ! It works well for lap joints but is not suitable for butt joints. Its strength comes from surface area whereas welding strength comes from fusion.

These days it seems to be used more as a decorative filler- large fillets, left exposed and clear coated. Not how you want to install replacement panels unless its a rat rod !!

i am a pro I-Car certified autobody tech.
the VW certified shop i work at has a welder that welds aluminum, silicon bronze and mig with the same unit. there are 3 different wire feeds and torch ends. it also electronically adjust itself and uses pulse tech for better welds..

this is the welder. i THINK the purchase price was around $20K. https://prospot.com/products/sp-5.3/

mig welding with silicon bronze is referred to a MIG BRAZING welding.

i weld patches with silicon bronze ALL the time. it works excellent for that. the best part is their is significantly less warpage with it. it also works fantastic for butt joint welding.
also I-Car recommends using silicon bronze for welding ultra high strength steel in certain areas. they also highly recommend using it when welding near ultra high strength steel.
also when when MIG Brazing use longer stick out to pre heat the wire so it flows under the panels for better adhesion.

i am not sure how you would set up a regular mig to weld silicon bronze properly. i can tell you using silicon bronze has faster wire speed with less voltage than regular mig welding. also when welding with silicon bronze the finish welds look significantly colder than a mig weld.

Silicone bronze mig brazing is not welding. It relies on capillary action not metal fusion. Typically you need a larger "root gap" to allow the filler material to flow around to the backside of the panel for strength. The process has less heat transfer and the main reason that oem's use it for hss and uhss in collision repair is to reduce the risk of further tempering the steel. Also has a less rigid joint when sectioning non structural panels. It works great for all repairs and is very strong when done properly. We have played around many times with metal coupons and then pulled them apart with the frame rack pulling tower (mig welding, spot welding, rivet bonding vs mig brazing etc). Also works well for filling in small holes if you damaged the repair panel with a stud puller or Lenco type dent machine. I use the pro-spot machine posted above and we have never had any known body filler failures. Like MrT mentioned the process is slightly different to traditional mig welding. Hold the gun further from the panel and do small tacks allowing to cool slightly between tacks. I'm an ICAR platinum steel structural technician.

https://www.youtube.com/watch?v=TQNpDQVc5wo

Just curious - how strong is the joint if you butt and grind down the weld flush with the surface?

The joint is very strong. A weld destruction test will usually show the metal tearing before the joint. ICAR welding classes require the metal to tear or have pullout for plugs in order to pass. Also keep in mind this is for oem repair procedures and all vehicle manufactures have different requirements. The ICAR stuff is overkill for any restoration type panel replacement, but you can do your own destructive test with some scrap sheet metal similar to the car. Cut a few note card sized pieces weld them together and bend back and forth in a vise. That's the easiest in shop test before welding on the vehicle. Practice different sized gaps and even backers to see how it works and how far you grind it down. It's surprising how strong it is with proper gap. This is an older article but it explains the process better.

https://rts.i-car.com/crn-137.html

This is good info. Thanks for the short article.