Haven't had that since the old FLYSURFER bags... I used a dry lube like a silicon spray to lightly coat them. Seemed to work...

I've got all plastic slider zippers. Zero problems with them. I had a couple of board bags with metal sliders and bought new zips and sewed them in. Pretty easy job and the bags and zips work like new.

About the only thing I found that worked with metal zips is to use them frequently. I only got corrosion on zips that had not been used for weeks/months. Since I started foiling that means my surfboard bags with metal zips. If they did get corroded then soaking the slider in boiling water freed them up. I've used spray on silicone lube. It didn't make much of a difference.

My wingding board came with a bag with metal sliders. Apparently YKK now have metal sliders designed for marine use (boat covers and clears and stuff). Hopefully the new bag has those sliders.

You can't just replace the metal sliders on zips with plastic sliders. Most board bags use coil zips and you can't get plastic sliders for coil zips. The other kind with the teeth are called Vislon zippers and you can get plastic sliders for them.

Using the zip regularly seems to be the only thing I've seen that works. Had some bags that I've been using for 4 years without a problem. No kiting for months in the first lockdown and suddenly gummed up.

So - get out and kite more. Will save you money in the long run