Dear all
Just working on the project based on carbon prepreg honeycomb structure. The procedure below is OK, the final rigidity is excelent, but there is a problem on visible surface goes to be non flat after vacuum bagging. See the procedure:
1. Hit the first  layer, carbon prepreg 210gsm, 1 layer, vacuum bagging, autoclave 100°C@-0.98bar/+8bar. Perfect results, excelent flat visible surface, no pinholes, final thickness cca 0,25-0.28mm.
2. Apply the epoxy glue RG (5min.) for better adhesive properties, put on the nomex honeycomb OX4.8, thickness 3mm.
3. Apply the outer layers on honeycomb side, 3x carbon prepreg 210gsm.
4. Vacuum bagging, autoclave 100°C@-0.98bar, no overpressure.
Final structure is fine, but on the mold side - the visible side of structure, there are after vacuuming around all the surface where the honeycomb was applied..how to say that..hollows? The surface is not absolutely flat like in step 1, but deffected by inner honeycomb core. See the picture. Any ideas what's wrong?
I'm convinced, the problem is vacuuming. If I hit the outer and inner side of prepregs separately in autoclave and glue the sandwich together in vacuum, the defect is quite same, if the thickness of outer visible side is 0.7mm instead 0.25mm, the defect is quite same.
Many thanks for support.
Rosta

It is print through of the core due to the heat and vacuum. Try using a lower vacuum, you don't really need full vacuum for bonding core you just need enough to hold everything in place - normally around 50% vacuum.

I would also suggest not sending it in straight for a 100° post cure, as this is essentially a free-standing post cure and should be ramped quite slowly even though the skins are already cured. The skins are quite thin and easily affected by the shrinkage of the adhesive. I find you will get this even with 2mm thick skins if you aren't careful. Are you sure the adhesive you are using can actually handle 100°? Most 5 minutes epoxies can't handle that temperature and would be shrinking pretty violently. If yours can, I would still suggest it needs to be ramped up slower.

Hanaldo, for better understanding, in step 3 the prepregs are not already cured (In this case if the structure isn't flat panel, needs two mould - inner skin / outer skin and bond together). If the shape is quite comlicated I put the outer skin prepreg not cured and full vacuum bagging for best copy the surfaces.
You right, the  epoxi I have used has temperature resistance cca 60-70°C (in accordance with TDS), but I think that's no reason why the core print through...