If the aluminum you are using is roof flashing, the butyl tape may not work so well and the metal is not stiff enough. Even 1/16" butyl if you can find it may not work exactly right. I only say this as a preconceived thought as I have used a lot of 1/8" and 3/16" thick butyl. You want to the butyl to stick and flow a little. With 1/16" thick, if it flows 1/32" well you only have 1/32" left and going over a bump of anything might be a problem.
If you are using aluminum plate 1/16" thick, then yes 1/8" butyl would work OK.
If you want to caulk, I suggest Dicor non leveling sealant. Click under spec, scroll down, they call it non sag for vertical surfaces. It is sold in camping stores, online in a lot of places. They make leveling and non leveling. https://dicorproducts.com/catalog/ro...-lap-sealants/
I have used the Dicor a lot, works well, does not harden, stay flexible. And you can get it off. A little heat from a hair dryer and you can scrape and peel it right off.
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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