What can I use to thin Bondo or another type of filler such as Squadron green/white putty. I want to make some weld seams on a armor project and my idea was to use a hypodermic needle to squirt a thinned bead of putty along the area then put the seam detail in to it.
You can thin Squadron Green/White with lacquer thinner or just about any of the common liquid cements.
I use acetone (nail pholish remover) to thin down putty (mostly Tamiya putty).
For real thin putty/filler;try mr surfacer 1200, it can be brushed on to maybe a seringe would work to…
I use NON-ACETONE nail polish remover to thin Squadron putty. That will eliminate the chance of damaging the plastic.
Bondo is completely different than the other fillers you mention. The other fillers are a solid dissolved in a solvent. Lacquer thinner is good for many of these solvent based fillers. Bondo, however, is a catalyzed polymer where a catalyst is added to harden it. You must thin the resin with an appropriate solvent BEFORE adding the catalyst, and these solvents are not as widely available.
i made weld seams on my 1/35 LCM using squadron putty only because i hadn’t discovered red bondo. i thought the welds were mold seams and sanded them off before realizing my error. i taped either side of the weld with the gap between the tape the width i wanted. (similar technique to doing seam filling on barrels). i filled the gap with putty, packed it tight, pulled the tape off, and knocked it down with nail polish remover. of course this was a flat surface but still. i use acetone or nail ppolish remover on red bondo with no problems. 
