I had the same issue and concern about the seams on the cushions
When I replaced my bed/dinette foam, as I remember the conversation,
the owner of the foam shop told me that there were two reasons for this.
He said that (1) the foam was not a terribly snug fit inside the cover and
(2) they left the slippery plastic film on the foam after insertion into the cover.
With the existing cushions, he said that you could open the zipper and
reach inside and grab a handful (or two or ???) of that slippery plastic
and just rip it loose and remove it from the cover.
To get the foam inside those smallish zippers, I guess that most everyone
uses some thin plastic (even a trash bag might do if you were doing this
yourself) around the cushion and then applies suction (like a shop vac)
to a single opening in that plastic. That sucks the whole foam cushion
block down to about one third of its original size and makes it relatively
easy to insert into that zippered opening. Once the foam is inside the
cover, you simply allow the air to reenter the plastic and the cushion
expands back to full size.
In the following thread, Carol H suggests that leaving the plastic might
offer some protection from soiling due to spills or ??? (Maybe you
could just remove the plastic from the sides and leave some plastic
on the larger flat surfaces or maybe simply using Scotchguard (as I
did) on the cushions surfaces would offer enough protection from soiling?
You will have to decide for yourself.
New Scamp "hard cushion" remedies
The new N36 foam (that I put in) not only deflects more at the shoulders
and hips but is also snugger fit. I have not yet pulled all (or lots) of the
plastic off of the new foam but, if the seams still slip around, that will
likely be my next move.
The owner of the foam shop also told me that, if I really wanted to stay
with those original harder cushions, that the best thing to do might be
to stomp on it
or put the cushion between two clean sheets of plywood
and run over it with a truck (or car).
The idea seems to be that you would have to crush
some of those more
rigid foam cells; just sleeping on the foam cushions is unlikely to do that
... or at least do it very fast.
When you put a cushion on the clean living room floor and stomp on it,
your entire body weight
is concentrated into a very few square inches
on the bottoms of your feet and you truly crush the foam in small areas.
When you just sleep on the cushions, your body weight
is spread over
many more square inches and, aside from sitting on the cushions or
kneeling on the bed, you are unlikely to sink in very far and crush many
of those rigid cells.
When I stomped on my old cushions, they did seem to soften up a bit.
However, I decided to get the higher quality foam and the snugger fit.
If the new foam is still a little uncomfortable, I will still have the option
of adding a mattress topper in the future.
That's what I think that I have learned so far.
I hope it helps!
As always, YMMV.