An often seen mistake we see on the show field is the use of black weatherstrip adhesive on classic Mustangs and Shelby's
Originally a yellow brush applied product was used. We can see evidence of it on original cars around the door weatherstripping, trunk weatherstripping and even the luggage protector strip on the drivers side wheel tub pinchweld in the trunk.
Not all of them are messy originally, but typically they never "stayed within the lines" all the times on any car. These pictures just illustrate those incidents.
Many of us have used a product sold by HERCO as a neoprene rubber contact adhesive
I find it easiest to purchase on EBay under the heading
1 Pint Neoprene Rubber Contact AdhesiveIt comes in a metal can with an application brush inside the can attached to the lid
Many of us have found that if left in the original container with the bush that comes attached to the lid, that the brush rusts and in turn contaminates the glue turning it a medium brown, so we have been removing the brush from the lid when we receive the product
Though the product will darken over time to an off brown it looks bad to apply it in this stat. Recently I found that the container itself contributed to the change of color (with brush removed) so when I received the last bottle I receive immediately transferred all the liquid to a glass jar.
Lastly if not sealed well it will harden in the container so seal it well and purchase only what you need for the job your working on - what has worked the best for me.
As always - hope this helps and saves some of you time and effort