Cost.  Work.  Messiness.  While fiberglass isn't terribly expensive, it isn't cheap either.  You need a lot more fiberglass and a lot more resin to do the whole thing.  And the stuff is a bit of a tar baby to work with.  Getting it to lay down nice and evenly and take the resin and not come up with the brush, getting everything on before the resin hardens.  If you're going for a classic series battered and beat up box it actually lends to the look.  But if you want a clean sharp Smith or Capaldi box it takes some skill and some work. 

I glassed the entire roof on my box.  I had some old fiberglass materials laying around from some other project that never happened and I finally got tired of dealing with rot on the inside.  I used up my materials before I got as far as I'd have liked, and it still leaks a bit, but it is much better now.  And it is much stronger, structurally.  That's actually what I'd do.  Rather than do the seams on the inside, I'd just do a shell over the entire exterior.   

Hi there, like everyone else has said I'd definitely be looking to fibre glass the entire roof for an out side box, got to be the best idea in the long run.

And so she rises from the ashes....Well not quite. At least its beginning to look like a Tardis again. It was very sad to see just the paving slabs underneath once I had taken all the old material off, it was like it had dematerialised.



Due to the way I have had to fix the corner posts to the base, I have had to lose about 4 inches of height. This means I will have to cut a bit of the doors off. Oh well, the joys of building with no real plan and hoping it all comes together in the end!

I have decided to follow the advice on here, thanks, and fibreglass the whole roof. It wasn't as scary as I thought it was going to be and I would like the Mark II tardis to have a bit more longevity. Just need the temperature outside to warm up a bit...

OOH!  Good point!  I first worked with fiberglass on my A-Team van.  I needed little bits to join the air dam to the fender flares and I couldn't find them for sale anywhere.  Studied up on how to lay fiberglass.  But I missed the bit where it said it had to be 60 degrees F to cure (I think that's the temp) so my first effort was less than successful.  However IIRC, once the temp warmed up it did eventually cure.  But there were definitely subsequent coats and much sanding and filling with body filler and the bits still look a bit like some clay children's pottery project if you look at them closely enough. 

One other aside on fiberglass:  Everyone--literally everyone--will tell you how bad it smells.  So to my mind this meant raw sewage/rotten eggs.  But it smells like airplane glue.  As someone with many happy childhood memories, putting together model WWII fighter planes, fiberglass resin is actually a pleasant smell.