DEAP-3600 is a dark matter experiment using 3.3 tonnes of liquid argon as a scintillation target to directly detect Weakly Interacting Massive Particles (WIMPs), a dark matter candidate. Mitigating background sources is crucial to dark matter searches. A large background model contribution comes from attenuated alphas originating from 210Po decays within the acrylic vessel surfaces. Alphas from decays within the acrylic inner vessel and from the acrylic neck flowguide are analyzed. The activity of the inner vessel is separated into surface and bulk components, and determined to be 0.22±0.02mBq/m2 and 3.68±0.06mBq. An event rate of 53.5+30−4.6µHz is found for alphas originating from the neck flowguide. An optimized event selection is obtained, making use of machine-learning algorithms to reject neck flowguide alphas and maximize WIMP sensitivity. In 802 live-days of DEAP-3600 data, the expected upper limit on the spin-independent WIMP-nucleon interaction cross-section is 1.9×10−45cm2 (90% C.L.) for a 100GeV/c2 WIMP mass.