So I wanted to fill a bunch of holes on the nose of the bus where all the trim strips were screwed in, and the mounting holes for the letters. I thought I'd share with you a technique that doesn't require a welder to make a reasonably good metal fill.
The technique is to solder! All you need is a propane torch (like you'd use for sweating pipes together) and some solder. You can use rosin core or lead free, but I've found that rosin core lead/tin mix has the best adhesion.
There are a few reasons for doing this technique instead of a mig or tig welder:
1) welding on galvanized metal sucks
2) not easy to get a backing spoon to weld the holes closed
3) not sure what else is behind there (plastic? coolant lines? wires?)
4) don't want to kill the coating on the back side of the metal, which would happen if you welded it. Less chance with a lower temperature process.
5) heat from mig or tig could catastrophically bow or oil can the sheet metal
So, use a flap disc grinder or a sander to sand down a small patch to the metal. On a bus, you'll need to go past the galvanizing as well.
Then, paint the hole with some flux (same stuff you'd prep copper pipe for) if you've got rosin core, you don't really need to do this but it helps a little bit with etch cleaning and visualizing if it's hot enough.
Use the torch to heat around the metal - don't apply the solder yet or you'll just waste it, because it melts really quickly.
When the flux boils away leaving a lighter residue, and the metal seems to change color a bit, push your solder into the metal just above the hole. It should quickly melt and pool up, sticking to the steel.
Work your way around the hole, wetting the steel with the solder. It'll cool down a little bit, so a quick touch with the torch to get it hot again. The solder should flow freely.
If it's too hot, the solder just all falls off.
The trick is to "paint" a little bit with the solder, playing with the heat and solder. When you get it right, you'll get some surface tension from the solder skinning the gap. When it does this, take your heat away!
If you get it right, you are done. Sometimes it leaves a gap in the corner, so you need to sort of re-do it. Use the properties of solder to help you. You can make it crunchy or flow easily to load up the hole, then a little pass with the torch to melt things down.
One of the nice aspects of doing this is it kills the little bit of rust inside the holes where the old fastener was at.
Once you've got all your holes filled, just grind them flush with the flap disc. Usually you'll need to make a tiny smear of body filler to make things perfectly smooth for paint.
If I had someone else to hold a camera I could make a video so the photos will have to do for now. I'll take some after photos with the body filler then primer on there and all sanded flat.