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Discussion Starter #1
I have come to a stand still. I am so scared to pic the wrong adhesives and sealers for my sonotube subs because the end caps were custom and cost a lot of money.

I will list what i need to seal and the challenges and finally some things i considered.

1. Top end cap to 8" port
Challenge A. - The port overlaps a hole in the top of the endcap but the port is not perfect so it does not meet the endcap flush in all places IE there might be some gaps.
Challenge b. - The port is metal and the endcaps have a high gloss automotive cleat gloss paint job on them.

My thoughts were use liquid nails mirror adhesive then seal the inside with spray foam - I am a bit scared of the mirror advesive because of possible VOC content and the fact that is says allow one week before sealing in when used on mirrors. Would Expoxy be safer? Also I have never seen anyone use spray foam as a sealant and wondered why?

2.Bottom and top end caps to the sonotunes
Challenge A. - The custom end caps are maybe under a quarter inch small or less but luckily the outside lip more than covers the difference

The challenge is that once I center it the sides of the endcaps that were meant to attach to the sides of the tube will have small gaps that I could use ahdesive on BUT the primary or initial hold will have to be applied the the underside of the outside lip.

Is there an adhesive I could use that would safely fill this small gap rather than relying on this cumbersome 2 step solution?

3. Sonotube being used as a port (8") to the metal port

Again there is not a perfect fit so while there is one small point of true contact the rest has slight gaps to be filled, it also has to be strong enough for metal and being used in a vibrating port (although 15hz tune wont vibrate that often ha)
 

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Of course no guarantee, but the polyurethane construction adhesive I used on my mal-x build is super tough, but messy. Label states adhesion to metal, along with most anything else. I think my build pic shows how the bond withstood stomping while the sonotube layers separated (it was a 24" sonotube). I believe you would be hard pressed to get better adhesive ~$4 ea

I laid a nice bead close to the edge of the tube all around, smooth it a bit with my latex gloved finger, then laid a bead on the endcap and spread it even thinner. When I slid the tube over the endcap, it squeezed alot of adhesive up and out as well. I then wiped it off outside.
 

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When I need to fill gaps and stick stuff together I use liquid nails construction adhesive or epoxy. Liquid nails over epoxy if you have an issue with it running after you apply it which it sounds like you might have.
 

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Discussion Starter #5
I found pl premium polyurethate construction adhesive which is supposed to be very good and a nuetral cure silicone to put over it for extra airproofing...

Sound good?
 

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Discussion Starter #7
Silicone will be on the inside of the tube.

I figure used the poly con ahdesive because it will help fill in any gas and to the real holding then climb in the tube and silicone around the inside and over the glue that sticks out ect.
 

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I found pl premium polyurethate construction adhesive which is supposed to be very good and a nuetral cure silicone to put over it for extra airproofing...

Sound good?
I don't think you'll find anything better than this stuff- it's what I used too. I would use glue blocks around the perimeter of the tube where it meets the endcap. Use the poly glue there as well. With many of these supportive blocks you'll have a stronger union. You could also cut small spacers to fit that 1/4" gap to center your tube, with enough of those you would use less glue and have a greater physical bond as well. If you know someone with a bandsaw or table saw, these would be easy to make in number.
 

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glue blocks are wood that you use to strengthen a corner or joint. By placing a square or triangle shaped piece in the 90 deg angle, the two adjacent sides can be glued, thus providing greater rigidity. You'll see these in furniture, often on the underside, unseen. To use them effectively (stronger bond), you want to have enough surface area glued on each side and the block needs to be square to minimize gaps. Of course, if your driver is right next to the edge, this technique won't be possible, but an couple inches on the endcap with a few on the tube would be ideal (more against the tube because it's thinner and can deform).
 
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