Should latex or oil based primer be used over lead paint that has been scraped and sanded, but not down to the bare wood.
Use an oil base primer. This will stick and seal really good. Bare areas (exposed wood) might need 2 coats, the first will soak in. If you use a standard oil base wood primer mix in a little Japan Driers to help it dry quicker, a cap full per quart is plenty. You can find this at any good paint store.