I don't know why they sell this stuff. I used it to seam seal my car last year and now I'm going through and removing all of it. It dries up so hard that it eventually cracks. I'm just glad I never painted the car since applying it.

Fast and Firm is typically only used on drip rail groove. A light coat smothed out with a finger.
There are other products for the seams of the floor pan inside the car and trunk

SEM 39377 is the stuff. remains flexible, non cracking. different sealers for different apps.

3M Fast n Firm is pure garbage no matter where you use it,especially in drip rails,the last thing you want to deal with on year old paint job.

3M needs to stop producing this failure of a product


OP,use 3M 8300 ,you won't have any issues,I use it all the time

This is the ultimate in 3M seam sealers. It's paintable, never shrinks, stays flexible and bonds to anything clean.
Winzer Products markets the same stuff. I've used them since the 90's with no failures. These are the only seam sealers that we use.


http://www.autobodytoolmart.com/3m-ultrapro-urethane-seam-sealer-gray-08361-p-10388.aspx

Steve