I have been thinning with Toluene it works great. Xylol or xylene is sold at big box stores as an alternate to Toluene but it does not work as well.

Be very careful when thinning adhesives. The chemical imbalance can cause the glue to loose its ability to bond like it was originally designed.

If you try adding some fresh adhesive to the old, it may help solve the problem of spraying. Otherwise just use the older glue with a brush and only spray fresh adhesive.

10% lacquer thinner works fine for weldwood. It sprays finer for certain jobs that you need to have a smooth glue layer. I don't thin for carpet for example but do for vinyl. It just evaporates and has no ill effects.

I use Toluene as well. speeds up flash time. Less spitting.

Toluene is probably more correct for thinning but I don't have any tolulene, I have lacquer thinner lol. I use lacquer thinner for my paint gun or when I only have that one crusty glue brush that needs softening to finish a job, so it's always on hand and that's what the glue gun gets too.
I'm pretty sure any solvent that ends with "ene" is really bad for you too.

I use acetone haven't had any ill effects yet.

I have been using lacquer for thinning and Naphtha for clean-up for years. "A little dab will do ya", if you're old enough to remember that phrase. :cool:

Yes, I'm hip to the Brill cream.

I have used Acitone without any issues.