Seam sealer for gutter?

[rockyrivermark]

1970 c10
So I'm working on the cab and struggling with the gutters.
My buddy works for 3M and gave me some 2 part seam sealer to try.
It was a cartridge and pumped through a air powered gun with mixing tip.
Anyway. The problem is that it's bumpy and pretty bad looking. You couldn't really smooth it off with your finger much without making it worse.
I did sand it to try and get it better but it's just not coming out that great.
Being a cabinet maker and caulking countertops often I'm quiet confident with that unless the caulk is too thick
Any thoughts on what you've used and the results greatly appreciated.
I can get it out with wire wheel on hand grinder if needed to start over.
Thanks!

Mark
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