One thing I have learned is sanding down metal takes FOREVER. After we welded up some of those holes, I had to use an air grinder and angle grinder to sand off all the old paint inside the tub of my Jeep so we could prime and paint it with bed liner. The tub is the entire inside piece that makes up the floor and wheel wells of the rear tires. After I finally got the tub all sanded, we primered the whole thing. After that dried, we painted on the bed liner. For the bed liner we had to roll it on with a stiff sponge roller so it would have texture. We ended up painting on two layers over two days.
This is awesome! Interesting stuff too!