I am trying to fix old metal barn roofs, I am coming across problems?

First off, is there an easy method to scabbing ( adding boards to the rafters for extra support )? I have tried a few couple things, but I think I am taking the long way, I need to know the trick to adding support. Any special things in Home Depot to help me?

Also a good cheap thick coating that can be painted over, the coating has to be able to cover small holes one or 2 inch diameter.

Lastly, I hope, anything that'll speed the job, but also produce minably good results, that'll last a couple years, certain screws, certain wood, ceratin tools?

OH, I do painting so I have a compressor, is there nails for a nail gun that's made for this? I use screws for the obvious durabilty over standard hammer and nails.... my e-mail address is zoomzoom2222@aol.com.... if you can please e-mail me