Well here is what I would have done... I would have first gone over the entire bare metal of the car with epoxy primer (I only use SPI Epoxy primer and their clears). Then you can lay your filler over top of the epoxy without even sanding it to adhere (SPI epoxy doesn't need to be sanded in order to put filler over it and priming first is highly recommended). Some people think bare metal is the best way to go and this is what I used to do until I started using SPI epoxy. Anyways after that, you can use a basic high or medium build primer or even the SPI epoxy on top. You can also reduce this primer and use it as a sealer as well.