I have used the silly putty technique myself on 1:35 scale armour and it works well. It does not give a very hard edge as there is a bit of softness to it but it depends how hard you push the stuff onto the tank how soft the line is (worth experimenting on a piece of scrap if you are worried). It is actually blue tak I use as we do not get silly putty in the UK. As long as it is not plasticine or any other petroleum based putty it is ok though - play-doh also works but I hate the smell of it but used it so the kids could net get it to tread into the carpets and furniture [:@] it is all gone now though
Cheers
Gorehound