Made some body parts, Need some advice Options

[TargaToy]

Had to create the transition from the driver's door threshold to the striker plate as well as the leading edge of the front fender. I didn't realize that latter portion was nothing but braze and bondo until I started grinding the paint back to get my cut lines.

The lower section of the rear fender (directly under the sail) is still free from the car. I screwed and clamped it in place while I made the pieces pictured. Once it's welded back onto the car, I am going to have very limited access to the back side of the repairs.

What I WANT to do is treat/seal the back side of the repair which will be the most prone to be hit with moisture/debris from the tire.








That said, here's my issue:

I do not yet own a paint gun, nor epoxy primer. I'm going to Ospho the back side of this area and then I hoped to prime, seam seal, and undercoat it (do not care about not having body color back there--just want "tough") with the fender still unattached to the car.

I know it's frowned upon but is there something in a rattle or brushable product I could treat the area with to make it impervious to the elememts later?