How do I remove all of the glue in gray headlight housing?

I separated a set of head lights to do HID retrofit.

I want to replace all of the old glue with 3M Window Weld in the channel of the gray housing. Plus the old glue is all lumpy so it might not seal well if I leave it there.

I'm having a very difficult time removing all of the old glue. After the housing comes out of the oven, I have only 20-30 seconds of working time on the old glue until it turns hard enough where I can't scrape again. I don't have the patience to wait 10 minutes to heat up and only 20-30 seconds of working time because it will take forever.

Is there a better way to do this?

 

$12 heat gun from Harbor Freight or your wife/girlfriend's hair dryer when they aren't home, and be careful not to overheat the plastic

 

Heat gun