i just built my box but theres some gaps, and advice on what i should use, i would love to just spray something inside that would put a coat over the entire inside, but also fill some of the bigger cracks. some are tiny, some are kinda big.

 

I would fiber glass the inside gaps. if you don't have any experience with fiber glass buy a body filler that has fiber glass hair in it. that would be your best bet.
good luck

 

Use Body filler its what ive used in all my bulids and works great.

 

Is there something else that's not as messy like some kind of liquid deck sealer or something that can be used instead?

 

Yes PL400 works great too.

 

Thanks mang!

 

i sprayed my entire box with rubberized undercoating, then even if sealed i sillicone in all the inner corners. she pounds hard now with no leakes.

 

Undercoating will seal but only on the outside, not the inside.

 

Just use resin mix it hot and it will flow into the cracks and dry fast. Can also be used on the out side.