For those curious how this turned out...
This Mini Traveller (with GRP upper rear van sides) has now had 1.3mm KilMat installed on the doorskins, the door pockets, the rear lower side panels, the bulkhead (above the shelf only) and the shelf itself. The floors have had 3.5"-wide strips stuck full-length along the tops of all the ridges. The original carpet / 4-way-stretch was laid / glued back over the top of the KilMat afterwards.
The roof and the upper rear side panels haven't been treated (yet) due to the considerable work of recarpeting them after. And the lower bulkhead/toeboard/interior wheel arches haven't been done because their shape doesn't really allow them to resonate anyway. The spare wheelwell and the area around it are too fiddly to treat. If I was to treat the roof in future, I'd probably stump up the extra for Dynamat just in that area, in the hope that the extra £££ would buy me the strongest and most heat-resistant adhesive.
KilMat applied onto bare paint sticks well. On tricky surfaces, a light application of spray adhesive massively improves adhesion.
The effect is impressive. The noise - in what was virtually a bare car - used to be apocalyptic. It was ear-damage-inducing even with ear-plugs plus ear-defenders. Now it can be driven on short trips with no hearing protection. And on longer trips a pair of foam earplugs makes it quite comfortable. No doubt doing the tunnel and the remaining large panels, and adding plenty of underlay on the floor, would make another big difference. But this is good enough for my needs. And anyway there are massive sound-passages in the bulkhead (the plastic speedo cowl and flexible fresh air pipe) through which plenty of engine noise can penetrate, so there's a hard limit on how much difference treating the panels can make.
I bought 50 sq ft of the KilMat. About 1/3rd of it is left over - which should be enough to to do the upper rear side panels when I get around to it. Amazon seemed to be the cheapest place to buy it. KilMat is about half the price of Dynamat and comes in two thicknesses - I used the thinnest. A roll of aluminium sticky-back tape is useful for covering joints and edges, so the black butyl can't bleed through the 4-way-stretch glued on top of it. And a small application roller (say 1" width) makes bonding it much easier - though a 1" putty-knife can be used instead, with care, to get similar results.
If the KilMat adhesive fails in this summer's heat (dream on) I'll update this thread.
Edited by alpder, 12 April 2024 - 09:16 PM.