Determining AL filler metal to buy for repair job

So the repair job on the 53' reefer is coming up, decking and some brackets. I know its aluminum, but that's about it. I don't know if this fits better in the commercial repairs section or TIG section, please move if needed.

How in the heck do I determine what type of aluminum it is or is there just a rod that will work? Or should I pick up a couple and see where each gets me?

I haven't seen the job yet, but I'm told it's 1/8-3/16" thick, welded (so I know it can be welded) and has sustained damage due to fatigue and impact. There are some cracks and I believe a puncture or two (or a dent that cracked, not sure). It will be fix what can be fixed and replace what can't.

Heading to the LWS tomorrow or Sat to pick up a bottle and some other stuff, some 309 for a cooling system piece, some tungsten in a bigger size, maybe a pair of gloves. Got the outlet and plug end today, electrician friend picked them up, should be burning metal by the weekend!

use ER4043 since welds more easy and flex's weld better since has silicon in it . this is were its nice to have a aluminum mig spool gun , noticed you have a Atlas 12" engine lathe I have a 14 atlas lathe made in 1936 lol

On the parts I weld, 5356 welds a little better than 4043 on the Syncrowave 250 and 250EX. 5356 welds very shinny and 4043 looks very lightly sandblasted compared to 5356. It would be worth trying both fillers.

Like recommended, I would say get both as well. I normally would start with 4043, but they both have there place and worth having in hand. The 5356 is more rigid, magnesium base, the 4043 is a silicon based.

on a trailer, as a rule your extrusions will be 6000 series al and your sheet stock will be 5000 series al. take it from there.