Richard,
The more the better.  The longer the better.  Practically speaking 
you do what you can within reality.  Years ago on 160 meters a friend 
and I would plant a new radial every so often and he could see 
definite signal increases on my signal until I had a half dozen or so 
wires out.  After that I guess it became academic - the S meter no 
longer saw improvement but I am sure that there was some theoretical 
difference.  Enough for scientific studies!!
Solid/not solid shoulnd't make any difference.  Good luck.
Pat, K9JAUAt 07:12 AM 7/2/2014, you wrote:
>Hi!
>
>
>
>Soon, I will install a 43' HF (160M - 6M) antenna.  I have a few questions
>about setting up the radials for this antenna, as follows:
>
>
>
>1.  The manual for the antenna recommends using 18 gauge wire.  Would using
>a larger gauge wire (e.g., 16 gauge( be better?  Since my radial system will
>not be ideal (see below), I wonder if increasing the gauge of the wire might
>be helpful.
>
>
>
>2.  Does it matter if the wire is solid copper or braided copper?  Is one
>better than the other?  Also, should the wire be a) insulated or b)
>uninsulated?  Does it matter, or is one better than the other?
>
>
>
>3.  These radials will not be buried in small trenches in the ground.
>Instead, the end of each radial will be "staked" into the ground, and also
>likely a few "stakes" along each radial's length.  So, regarding the
>"stakes", is it OK that they are metal?  Is it better if these stakes are
>insulated, similar to the wire?  Would plastic or wood stakes be any or much
>better?
>
>
>
>I think I will be able to layout about 16 radials.  However, not each will
>be the height of the antenna (maybe half of them will be), their layout will
>not be symmetrical (like spokes on a wheel) and about 180 degrees radius off
>of the antenna will not have radials extending outward in this quadrant.
>
>
>
>Finally, are there any tricks I can employ to improve my radial system?  For
>example, I think I once heard of someone laying down chicken wire around the
>antenna base.  Is something like that beneficial?
>
>
>
>73,
>
>Richard KK6MRH