MGB Stopping rust under patch panels

[DCKmgb]

I have been using a weld-thru primer for patch panels and rust repair and recently notice that I am having some rust under welds at seams, even though weld-thru primer was used.
Are there any tricks or other types of products that I should use under welds.

Note: that this area had not been in the weather outside at all, always protected.:grumpy::grumpy:

 

[Bob Branson]

Barney uses Chassis Saver, so I bought some. It's a coal-based product, very thick and dries rock-hard. About $90/gallon online.