Cleaning up  the engine bay in my 73.  Between the firewall and fender and between fender and the rail there is a gap.  Was this previously filled in with a rubber caulk like material?.  I have been power washing and may have washed some of it out.  so question would be what to go back with.  There is a 3m marine product 3200 and 3600 that is rubbery.  the 3600 material is almost not removable.  

 

I don't believe it was sealed from the factory. Which is why they rust out there. I wouldn't use any of the marine caulk products. I would get a dedicated seam sealer and use that. The seam sealer will take paint. I would also try to treat any rust that is down in there first

⬆️ ? automotive seam sealer.

My current plan is to clean off as much of that rust as I can and then go back with POR15.  That POR15 should wet down into those open seams to stop the rust in there.  I'll end up with a black engine bay unless I go back and paint over it.  I've got the same problem  under the battery platform.