Someone out there, maybe Eastwood, sells the wand thingy. It's part of an undercoating gun, which would be ideal for this application.
Short of building it from scratch and then pressure testing it with soapy water I don't think you'll ever get a boxed frame liquid tight.
Short of building it from scratch and then pressure testing it with soapy water I don't think you'll ever get a boxed frame liquid tight.

