Hi Randy

 

I wouldn't worry about the prepreg, just do some wet layup over balsa or foam.  You can get select aircraft grade balsa that is very light, I have some that I used for about 4.5 lbs/cu. ft. I'd part with it but it is too thick at 34/".  I would suggest you use Baltek's AL-600 pre-coated balsa to ensure that you don't absorb too much resin.  It is very common to have too much resin soaked up by the balsa and have hardly any bond if you don't use the AL-600 or doa hot coat to seal the wood's cells. 

 

As to thickness, what you need is about 1/4" thick.  I should also mention you could also use foam.  Divinycell H-80 (5 lb cu. ft. would work well or even H-60 (3.5 lb cu. ft.) but it won't have quite the durability that balsa would have.  It will dent fairly easily, however, if you place carpet over the sheet, it won't be too bad.  The strength will be very little different between the balsa and the foam but the toughness will be very different.  And I would also suggest that one layer of 6.0 oz carbon with 1/4" - 3/8" core will be fine.  Remember that it will be attached on all 4 sides and thus any deflection will turn into tension pulling evenly on the supporting sides so the deflection and unsupported area is very small.  You could walk on that layup and not have it break. 

 

To keep the weight down, DON'T go with contoured foam or balsa (cut into squares and backed with fiberglass cheese cloth) if you can vacuum bag.  I would use flat sheet of core, laminate to a flat mold and vacuum bag directly to the back.  For a flat mold, 3/4" thick melaine finished particle board with a little wax works great.  First mark your areas to laminate, lay down the cloth and apply resin.  Next apply a little resin on to the back of the core, then place the core on the wet layup.  Next wet the top of the core with a damp layer of resin again, then layup on top of that with the 6.0 oz carbon.  Place the peel ply, then breather ply and last the bag over that.  Suck her up and you have a flat panel for not a lot of money.  You can also try Teklam (in Corona, Ca. I think).  They may have some panels ready to go.  They do all kinds of prepreg panels and may have something sitting on the shelf.  They are great folks.  I can get you the number at work if you  email me at dan.newland@orcon.com.

 

If you don't have a vacuum bag system ready to go, don't sweat it too much, just do a layup as I mentioned before but I would bond the core after the first skin has hardened by using resin thickened into a putty with cab-o-sil and a 1/8" notched trowel.  Use a rolling pin or a ruber mallet tapping on to a chunk of plywood to squish the core down into the wet goo.  After the adhesive hardened, I'd remove the "Cheese cloth" backing and apply the top layer of carbon and resin.  It shouldn't be a whole lot heavier than a vacuum bagged layup.  But one layer of carbon on each side of core should do fine and be all you need.

 

Dan