whats under your bumper cover.. this was mine as you can see it crumbled completely away prior to me buying the car but that was the least of my concerns and the seller included a new fiberglass cover for me. (Note the 80s radioshack horn solution he used with a bubba doorbell button rather than repairing the factory horn switch )
From the pictures your bumper paint does not match the car, this indicates it was replaced or repaired and repainted at one point already.
Honestly your putting time, $ and effort into trying make something that need replacement, look presentable thats really not hard to replace in an afternoon. Just an opinion here but to me, even unpainted fiberglass will look better and more respectable than trying to doctor up that bumper which will look to others more like you are trying to avoid proper repair than anything.
The bumper cover afterall is just a cover and cosmetic in nature on these cars because the real steel bumper is underneath. I drove my car a whole summer with all the paint scrapped off with a razor blade. which ironically worked out for me since that summer someone backed into it while parked at my nephews graduation party. (Its been hit twice now while parked, hence the bright paint color!) and their insurance paid for the materials for me to repair and paint my car.

To be clear here you CANNOT repair your existing bumper. The material it was made of has significantly dried up and shrunk and become very brittle. It was originally very flexible almost like rubber when new. Nothing will stick to it effectively and it will just continue to shrink and crumble. Be glad you dont have a 74 where you have 2 bumpers doing this and needing replacement. Both of mine are now fiberglass.