Spraying undercoating inside a sub box?

Would it be a good idea to spray the inside of my box that I'm building with a automotive undercoating/ sound deadning spray? How would this help or hurt?

Also, How about brushing on fiberglass reisen on MDF. Should I do the inside or outside of the box, or both?  How would this help or hurt?     I would think it would make it stronger?

What about polyfill? If my box was built to specs , is there a need for polyfill? What would happen if I added it?                   If the box I built ended up being slightly under specs then how much polyfill should I add? Say the box was under built by  .5 volume.

Last one: If I built a box just right "specs" for a sealed enclosure,  then wanted a ported box instead, could I add enough pollyfill to compensate for the volume needed in a ported?

Thanks for everyone's help!!!!

polyfil can increse the relative volume of the box by 20% alos porting it shouldnt need a much larger box if it needs to be any larger use more and longer ports