I got the same kind of results with RTV615 that Andrew did. If you're
spinning directly onto your part then you are ok, but if you need to
remove it, I'd suggest spinning on glass coated with acrylic enamel (spray
enamel or liquid). Then dunk the whole thing in ice water. Shrinks the
glass and expands the silicon - comes right off.

Any particular reason you don't use a photoresist like Shipley 5714? That
spins up to around 15 um thick or so. Don't remember the rpm off hand -
probably 2-3k.

Some tapes are also available around that thickness if you don't care to
much about exact placement.

Mike