is there any way a do it yourself diesel mechanic like me could repair the rust on the bottom of my door any suggestions its pretty bad

 

I did mine at first by slicing out all the rust and using fiberglass cloth with polyester resin in layers to make a new surface where the rust was. The problem with this method is its messier and requires re-repair of the whole area after a dent/crack. Something I found real useful was using sheet steel and wax paper as a mold to get a nice flat finish.
What I did this time around was to take a cutoff wheel and make some square cuts around the rust. Then get some heavy sheet steel and trim it down to a very good fit in the hole. Then use magnets or vice grips to hold it and tack it on with a MIG welder. Ive had very poor luck doing stitch welding with this. I ended up doing a stitch method but with individual spot welds until they overlapped.
Then all you do is grind the welds flat, fill with bondo, sand with a backing board and paint.
Im using some rust converter I bought from theruststore.com (its the tannic acid type) which works nicely on rust so far. You can also use a zinc-phosphorous solution to etch / rustproof the panels or use some zinc etch primer under a primer-sealer.
One last thing.. Id drill some drainage holes into the area at the bottom of the door sill and door skin so water doesnt collect.