Problem is NOT Pipe Cracks
Re-read the suggestions from previous post such as Whitespider ("Spidy") here.
IF your flue has good draft small cracks should not affect the draft or leak smoke. You have 'other' problems that inflexible furnace cement or any sealer will not solve. Metal pipe moves a lot.
Try:
1. Is the cap or top of the flue blocked -- dead birds, neighbor playing with you, creosote ?
2. Hot fires with dry splits and plenty of paper firestarter ( no banjo starters please like AV gas :hmm3grin2orange: ) ?
3. The "cracked door" makes sense IF the stove is older without good primary air control ---could be clogged or inoperative.
4. Is the "flue" too large for the stove e.g. stove specs 6" flue, butt you have it into a 12" square masonry chimney ?
How'd we do ? PM us for a bill.
Oh yeah: If the day is warm, damp with low barometric pressure, e.g. rain, a marginal flue will not draw well.