i've got a full length outer on the rear cable, but the front mech cable only has a short outer cable down to the stop on the downtube, and often gets sticky when i'm shifting if its muddy.

is it ok to run a full length outer to the front mech, or will the sharp bend where it goes under the BB and up the seat tube mean lots of friction?

If it was a full length outer would you need to run it under the BB?
Could it not run along the top of the downtube, then wrap around the side of the seat tube, more gentle bend that way?
Lots of cable ties though, could get creative with your bottle bosses if you have any.

i think its a bottom pull mech so would have to come in from underneath??

Edit – ignore me i'm being thick, i thought you meant run it along the top tube!!! I'll maybe try your method cheers

Yes, it would have to then. You can't use a longer cable in theory because there's no where to fix the end of the outer (eg no cable stop) though with a zip tie, you could in theory keep it in place on the seat tube. The problem I'd see there though is that any water/mud from the back wheel will hit it and run straight into the outer, collecting at the bottom so it's unlikely to work well in the long term. I'd suggest sticking with the standard routing.

No problems with my full length outer on my Stumpjumper to the front mech. Came from Spesh that way. Only issue I have had is the cable outer eating into the frame, now stop with a clear plastic patch. Got four cables under my BB, rear brake hose, front and rear mech cables and a Joplin remote cable.