I have a relatively small (maybe 5"x12") area that has gotten soft. I'm not crazy about ripping out the floor, the rest of it is fine. I hae done some research into Smith's Penetrating Epoxy, and it looks like a good option. Anyone ever use this producct?
Nope, but people do it with other epoxies (west systems, system 3, ect.).
If it is really rotted though, I would overcutout the area (maybe 10"x20"). Make rabbet cuts with your router around the cutout of the existing deck. Cut a new piece of wood to the dimesions of the rabbet cut and then rabbet cut the new piece of wood. The rabbet cuts will form a lip for the new piece of wood to sit, increase the bonding area of new to old wood, and will prevent the new piece of wood from falling thru.
I have a similar situation with two smallish soft spots. However, I fully assume that when i cut a section out, I'll find waterlogged floatation foam under at least some of the floor. My guess is that most rot comes from trapped wetness under the floor.
God luck - I hope it's as small an area as you think. I hope mine is also, but not counting on it.
When you cut make sure that you cut out all of the bad wood even if the hole ends up being considerably bigger than what you thought. Rot is a fungus and will continue to travel unless completely removed.