You need to remove all loose paint or chipping paint or alligatoring paint.
You can use a putty knife or sand or power wash.
If you thing its lead then do not sand. Just fill in the cracked or cracked removed paint chips with exterior spackling. This will make the final paint job look smooth.
I would apply a good quality primer like Zinser BIN. Let it dry then apply your final coats.
You should not have any problems.
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