Thinning silicone so I can pain the inside of something to water seal it?

You can't thin silicone. Use something else.

Don't seal it with silicone. Seal it with shellac. That paints on well, you can thin it with ethanol - which is a lot less toxic than acetone - and soaks into the pores to make a more durable seal.

Acetone or Methyl Ethyl Ketone will thin silicone to the point of becoming a penetrating oil. There once was a product that used mineral spirits to dissolve silicone, but it never dried really well. Bad deck coating. Work outside with my suggestions.

You can use epoxy for that. West System is what i use. You can also use salad bowl finish if the bowl is wood. I have also used bees wax and USP mineral oil for a totally non toxic finish. (Melt the beeswax and mix it into the oil).

You don't need to paint it on. Wear some disposable plastic gloves and rub it in with your fingers.