Prolong exposer to sea-stone drains devil fruit user's strength until they unable to freely control their powers and user recover immediately when not in sea-stone's effect, and we don't know just how brittle pure sea-stone is?. As usually it a mixture of metal and sea-stone. For logia sea-stone bullet will just pass through before user will lose his powers; same for Luffy, he will bounce them back before being exposed to effect enough.