Do I have to sand the 2 part primer before putting on the urethane seam sealant to help it stick better?¿? Thanks!¡!

You don't have to, but it helps when you have to sand it later anyway to paint. If you don't sand, then you will have to sand right up to the sealer and it may be damaged. Or at the very least it will be a pain in the tookas.

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1965 Buick Gran Sport Convertible
1965 Buick Skylark H/T
"Fan of most anything that moves human beings"

Makes sense, we will go for the pre-sand method! Thanks.