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Got this done yesterday using Peel and Seal from Lowe's. It may not be as thick as dynamat, but at $16 for a roll (I only used 1/3 of the roll here), I'd call it a win.
Now I just need to find a decent stereo to power the speakers...
I've been in quite a few "discussions" about big box store HVAC stuff vs purpose made like Dynamat. I'm from the group that believes you get what you pay for in this area. Having spent three decades in an industry dealing with adhesives of all types and their pros and cons and knowing the how and why the one that costs a dollar can't really compete against the one that costs $100 I have my opinion but get why many choose to try and save some money.
My only advice is if you do decide to use the HVAC stuff be sure to REALLY clean the mounting surface and spend some time working it down firmly. And then do it again. I also wouldn't put it under the roof panel under any circumstance. It is made to handle indoor air temperatures in terms of adhesive performance. Ever touch the roof of a car on hot and sunny summer day?
I put some Peel-N-Seal in my '67 over ten years ago. Never noticed any smell. It rarely sits out in the sun but not one piece has even started to come loose in ten years.
I put some similar speakers in my car. Went ahead and gave them their own amplifier.
Cleaned the surface with degreaser, then brake cleaner.
Installed with a heat gun and a big socket to roll the butyl onto the surface. Thats more heat than it would see even in 110F summers. No smell, and it's definitely not coming off.
I'm sure it's not as good as the purpose made stuff, but for $15 it does the job better than the disintegrating jute that was on there before. All I wanted was to keep it from rattling and maybe insulate the cabin from the trunk just a bit.
I do agree that it's absolutely not suited for underhood temps. The butyl starts to liquify and drip at those temps.
The speakers were essentially free. Nothing fancy, just something to replace the blown 6x9s in there when I bought the car. Next step is a stereo to power them.
Nothing wrong with saving a few bucks where you can. Sounds like you did a lot more than most in terms of surface prep and installation. Longevity is the real concern with adhesives. The basic type used in this stuff is pretty forgiving but age plays a role. For a crude example, think of old masking tape you may have applied to something and left on for a long time. Most masking tapes are designed to perform for only days with the expectation of actual use being much less. The HVAC stuff is meant to last years but again, in an indoor environment with a narrow range of temperature change. Getting cooked in the summers followed by months of freezing temps during winter is where the real potential for early failure lies. Use on flat surfaces like floors shouldn't be an issue it's when you get vertical or expect it to hold to the bottom of a roof is where all bets are off.
For me, I'd rather spend more on something that could require complete interior removal and a lot of work to remove and replace if it were to fail.
I don't know that butyl really cares much about temp cycles, though. It's meant for use on roofs, to basically form fit onto surfaces and not come off. It's not an adhesive applied to something, the butyl itself is the adhesive. It can melt, but it's tacky by nature.
Having played with this particular version, I'm convinced most of the problems people have result from poor prep and lazy installation. If you just slap it on a surface, Particularly a dirty surface, it's going to come off. But if you clean the surface well and apply with heat and pressure, it shouldn't come off. Shy of direct heat from the heat gun and a putty knife, I can't get it back off.