Sorry I forgot to reply this. Lets talk a bit a bout putty
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Miliput, I imagine you have bought miliput standard, witch is green, I hate that one, is quite rough, It smells really bad and is a little bit difficult to manipulate. I use that one to make skeletons and guts because is cheap.
Initial phase of mi hive ship, first tried to use miliput because is cheaper, but it just didn't result, specially if you are not very experience
Green stuff, you should buy it to make the "skin" of your bioships, if you use miliput your result will not be as good.
Here is very cheap.
http://www.ebay.es/itm/Green-Stuff-Kneadatite-36-inches-/310236159605?pt=UK_Toys_Wargames_RL&hash=item483b841e75After that I started adding green and this is what you obtain with green.
But that are by far not the only alternatives you have.
Miliput red is not the only one miliput offers, miliput black and miliput grey are quite better, but also more expensive, and you can mix miliput with green obtaining putty with a mixture of both capacities.
Also when I start my ships I prefer to use even cheaper materials as balsa wood and aluminium paper , and puttys like paper mass and recently I have discover fimo.
is quite cheap and the results are quite organic, of course you can not use them for the surface unless you are an artist
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squeleton of paper mass+aluminum paper.
squeleton of fimo and result of the second layer of green.
But there are quite more tipes of putty, for tyranid ships I don't think is worht it but If you are interested in other kind of modeling just ask me
I am welll documented