You still have copper so all is not lost. The "tinned"coating is just scaling off.
When boards do this they often also get the solder bubble effect which drives me crazy.... where you form a joint and as it cools down a bubble forms inside the solder and it pops leaving a crap joint. Reheat and add more solder, pops again, over until finally get a nice joint formed. No amount of even the most aggressive flux helps and too much makes it worse
Roughing up the copper with fine sand paper helps get the new solder to stick. Just dont sand through the copper. The copper will re-tarnish over again in days/hours so gotta solder before that happens.