Sash replacement--what about bottom of frames?

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