Hi mate, my underside is all painted and polished like you want and i'm sure mine was originally brush painted.

I've just re-done what the previous owner did as he had missed bits and I used a brush and normal 2k paint.

The finish is obviously nowhere near of that on the main bodywork but is a nice smooth paint finish and nice and bright white again, but without grinding off all the marks in the floorpan and filling them you never will get a finish like that.

If you've got a nice clean garage with a heater then that sounds ideal mate, get to it and get some pics up in the resto section!

HTH