I did a couple more things today working on the cowl vents from the inside of the car.
On the driver's side I painted a couple of coats of rust encapsulator all around the cowl vent. It looks like I have that completely coated now.
I did a trial fitting of the cowl vent insert. Although I have put seam sealer all round the inner cowl vent stack, the plastic insert still fits. I marked the insert to make it easier to orient and install. The plan is to put as much seam sealer around the plastic insert, then install it and then use fender washers and nuts to secure the insert so that its firmly installed.
I may end up having to get another tube of Dynatron 550 seam sealer just for this part.
I did not check the passenger side cowl vent.
