The high chemical reactivity of strontium, which can opacify a viewport exposed to a strontium atomic source, is a concern for some atomic physics experiments where it is sometimes needed to send a laser beam counter-propagating compared to the atomic beam. While a number of experiment use heated sapphire windows to reduce strontium deposition and increase the viewport lifetime, here we study another possibility, consisting in sending the laser beam onto the atomic flux by reflecting it on a mirror at 45$^{\circ}$ exposed to the strontium flux. We show that an SiO$_2$-protected reflective surface can be a viable solution. We find that for deposition of up to 1.5$\mu$m of strontium onto the mirror, a reaction with SiO$_2$ preserves a transparent medium, such that the high reflectivity from the back surface persists. This contrasts with deposition on inert sapphire, which makes a much poorer reflective surface. Our results provide guidance for Sr experiments involving Zeeman deceleration, and an alternative to the use of a heated sapphire viewport. We also provide a viability estimate depending on the atomic flux.