You can figure out which laquer thinner works best and how much to thin it by making a few very small (1/2 cup or so) sample batches of undercoating, and brush them out on some scrap metal leaving a final thickness of 1/8" or so. Let them dry.

You can try using automotive lacquer thinner, which should cut the material, flash off nicely and work. The colder the conditions, the faster evaporating the thinner should be. Do not use mineral spirits! It will never dry.

Try making a few batches that end up having a spray-able consistency through an undercoating spray gun similar to this.

https://nhoilundercoating.com/product/spray-gun-for-nh-oil-undercoating-products/