What about sustained attacks? How do those work with preps? 

I'll throw out Green's mech as an example, specifically its shields which are stated to be capable of stopping any un-prepped attack. What if someone was to hit them with an un-prepped sustained particle beam and then continue the beam attack for several posts? Would each successive post that the beam continues to hit the shield count as an extra prep that he would have to focus the shields to defend against, or would he be able to essentially ignore it because it still counts as an un-prepped attack no matter how long it's hitting the shield?