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

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

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LOW TONEZ AUDIO

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.

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LOW TONEZ AUDIO

Thanks mang!

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

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

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LOW TONEZ AUDIO

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

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1993 honda civic dx
Security: compustar 2way
Head Unit: Alpine 9886
Amps: MTX mxa6001, MTX TC4004
Wiring: stinger
Mids & Tweeters: mb quartz rc216
Sub: 12" ssd fi audio in custom ported box(2.6 cu ft) tuned to 32 Hz
SPL: 140.3 at the dash at 38hz(Door open)138.7 with music tested with a term lab mic