I have just bought some 20lb test Berkley whiplash and am wondering how to spool it onto my baitrunner 6000.The braid is so fine it looks as if 100m would be lost on top of the 20lb mono underneath.I have a 300yd spool but was hoping to load two spools with it.Is it worth taping over the mono to create a smoother bed for the braid, or do shimano make fine spools for my reel? Also I've read somewhere that you shouldn't fill the spool as close to the rim as you would with monofil.Any help please!

20 lbs whiplash pro is 0,20 mm thick if the tackle box has it right in their line-tests. It should be no problem to load it on top of your mono. I see that 0.06 is stated on some websites and then it might be a problem. You are correct in that you should not load it as close to the rim as monofil(to avoid serious tangles).
On earlier models of the shimano baitrunners you used to be able to put a 5000 spool on a 6000 baitrunner, you might want to check that if you want a smaller spool.
Edit: Tape should work also, see to it that you apply it evenly.

Should be OK. just make sure the mono is nice and tight with good line-lay before starting to load the braid. Strip 100 yards off and re-lay to be sure, you'd be surprised how irregular it can get when actively fishing.

The other thing I'd suggest is to use not less than 150 yards or meters of braid on a single spool over backing. I tried 100 yards but got interference with the knot used to join to the mono backing.

Because the braid is so fine comparatively speaking the knot joining one to the other didn't just get buried and forgotten like it does when adding mono to mono. It then became a real pain, catching the line in a pluck pluck pluck sensation as the line was coming off the spool. That extra 50 yards of braid on the spool buried the knot out of harms way and no probs.

Taping over might work but rather defeats the purpose of backing once the braid has been run out by the fish of a lifetime..... ! Perhaps run some plumbers ptfe tape on to the spool under the last ten feet of mono, will then fall off if you ever get that far down.... ?

Must confess I have run my braids as close to the edge of the spool as paralell, same as mono, with no problems. Being so limp braid doesn't 'ping off' in coils like mono and I have found it relatively tangle free in comparison - and if you do get a tangle then braid is 999% easier to undo than a mono snarl.

thanks for your replies gents, I can't wait to get fishing with it now!

Maybe another method of dealing with the knot problem would be to put a small piece of insulation tape over it on the spool....
That way covering up the bump and smoothing it out for line lay / casting purposes, but presenting no difficulty in pulling off the spool in the unlikely event of a fish running strong and long enough to get you down to the backing.... must try it myself !
Tho I think on reflection the knot thing is probably more of an issue with the thinner lower test braids, I'm generally using 8 lb stuff which is as thin as 1 or 2 lb mono....