You can go down to about 120 grit before the foam starts tearing. For a really smooth finish give it a quick pass with a heat gun on low setting but make sure you practice first on some scrap so you know how far away to hold it and how long to linger in one area. The heat gun melts any fuss left from the sand paper.

You still have plenty of grip from the foam and you don't have to worry about the lycra etc coming unglued.

Cheers
GrantG

: I cheated. I let Joe make the seat. I have since seen him demonstrat his
: technique and have replicated some of it in building the seatback for my
: new SOF. Start with a right-angle grinder with a 36 or 50 grit disc to do
: the gross shaping. A light touch and a strong grip on the grinder are
: critical, as foam disappears at a shocking rate. Never stop moving with
: the tool. When you get the basic shape you want, shape with a piece of
: sandpaper (60 or 80 grit) wrapped around a piece of minicell. That is
: enough for a reasonably smooth finish. You can use finer grit paper for a
: smoother finish. If you want it really smooth, apply spray adhesive and
: cover with lycra or speaker fabric. I like the grip of the 80 grit finish.

: Joe (Greenley) feel free to correct me if I have misspoke on the process.

: Regards,

: Marcel