Seam Seal Firewall
 

I'm in the process of seam sealing the firewall on my 67 GTO. How are you guys seam sealing the firewall to get clean straight edges? I masked it off but when I pull the tape off it wants to pull the sealer off the taped edges a little. I pulled the tape off as quickly as I could. But I think it is starting to skin over before I finish applying. I have tried 3M brushable and now I have some 3M urethane and Dynatron I want to try. Any suggestions?

Brushable
 

3M seam sealer brushable is for the floor inside and trunk panels where it will not really be seen.
3M make a white seal sealer in a tube that is water based until it dries. That is what the factory used on the firewall.
It is white and in the tube. You have to cut the tip wide to replicate what the factory did and be careful with application. Then use a wet rag to wet your fingers and flatten it out just a little to make it look correct.

Practice on an unseen area then do your firewall when you get the technique down.

3M 08500 is the part number

I don't know what you seam looked like originally but mine definitely wasn't clean and tidy, it was sloppy and looked like it was done in 10 seconds. It took four tries using a plastic spreader used in bodywork that I cut it up to mimic the original look on my car. Don't make it too nice it will look out of place.

It was originally done with a gun with a brush end hooked to a hose hooked to a 55 gallon drum and a pump. I started in the paint department at one of the big 3 in 1989 and used to apply seam sealer. To try to duplicate it now I would use one of the sealers in a caulk gun and drag a wetted paint brush through it immediately after caulking the seam. The brush on you have is really good too if you work fast. You could probably use a brush with a little bit of mineral spirits in it for that one. Rob is exactly right on the sloppy and 10 seconds. We weren't worried about how it looked. As long as it was sealed into the oven it went.