If I notice that the stamp is poor quality before I walk away from the
border and customers desk can I ask for a new stamp?
Yes, you can ask and yes, they would stamp a new one. In your case they would probably just stamp directly over it. You should always check your stamps upon receipt as mistakes do happen - especially if you cross the border after midnight as you might still receive the stamp on yesterday's date.
Could I have issues trying to leave the Portugal with a poor stamp?
For example, they won't let me leave because they can't validate the
seal or date or think the poor quality is suspicious
This is covered by Schengen Border Code article 11:
- If the travel document of a third-country national does not bear an entry stamp, the competent national authorities may presume that the holder does not fulfil, or no longer fulfils, the conditions of duration of stay applicable within the Member State concerned.
So if your stamp is unreadable, this could technically be interpreted as no (identifiable) entry stamp, and result in harsh consequences. However, here is your legal option:
- The presumption referred to in paragraph 1 may be rebutted where the third-country national provides, by any means, credible evidence, such as transport tickets or proof of his or her presence outside the territory of the Member States, that he or she has respected the conditions relating to the duration of a short stay.
Thus legally you can get yourself out of trouble if you keep and provide evidence listed above. Notably you can establish your arrival date in Portugal using the arrival boarding pass and evidence of your presence outside Schengen area (assuming this was your first flight there).
Practically, however, this is extremely unlikely if you're exiting passport control through Portugal, as it is not strict on stamp enforcement. You might have problems however if you exit through another country, such as Germany, Netherlands or Scandinavian countries, and for those you need receipts.