Thanks @MitchellMc, that makes sense and I have sanded a fair bit after you mentioned this. It's looking like it's already hardened more but 2 more we ...

Follow-up question @MitchellMc : I used the product a week ago and after it set, it seems to be pretty chalky/powdery and easy to sand. It's not an ...

Thanks @MitchellMc. I will try to get an assessment and see how it goes.Although, it just seems to be neglect over time (by previous owners/tenants).

Thanks @MitchellMc

Thanks @MitchellMc , I had stepped drill bits in mind, so that will be my next experiment. Also, do you recommend hole saws for this purpose?

Thanks for the warm welcome @EricL . I have attached pics of the flyscreen. As you can see, the wider inner track is for the sliding glass window. I'm ...