We're thinking of replacing the double-hung windows in our 1930s house
with Marvin Tilt-Pac sash replacements, since the frames are in good
shape. My only real worry is whether the seal will be tight at the
bottom, where the new sash meets the bottom of the original frame. The
bottom of the frame has peeling paint, etc., and I can't imagine that
even if we sand and re-paint the frame bottom, it will provide a good
seal when the new sash is closed. Has anyone had experience with Marvin
Tilt Pacs who can advise on this issue?

They've a flexible seal on bottom. Will cope with lots more
irregularity than peeling paint. Which I'd remedy ASAP anyhow.

I've 9 pair on one floor, wood sash.

Thanks very much. Can you describe "flexible seal" in more detail? Is
it like a rubber sweep? Any pictures? Thanks.