Buy yourself one with plastic zips (not just the zippered part). They are available for surfboards etc.. Have experienced same prob as you and havn't had problems since going for all plastic.... You may need to pay more.. Can't think of brand names at present.. sorry..

Yep. First avoid metal zip sliders (or whatever they are called) by always buying fully plastic ones. They are worth the extra dollars. Having said that I also have a couple of board bags that have metal sliders. They attract salt. Best to hose down the whole zip and board bag every now and then, but I don't do that

If they get stuck I pour a bit of water over the slider and jiggle it a bit - comes loose in a couple of jiggles.

Wash the salt off the zip is best but if you want you can then spray it with a plastic lubricant like the one they use to lubricate popup plastic sprinklers - get it at the retic shop

If they are left in the back shed for a season they are usually too rusted to recover

When I was sailmaking, we used to replace them all the time. Save the bags (probably too late now) and go to a sailmaker and get them to sew one in. Unless they're a tight ass, it should only cost $20 to have a brand new plastic zip in its place. Whats a new bag worth??

From personal experience, look at the actual tooth size of the zip. The bigger the better when it comes to avoiding this sort of problem.

EDIT: Try silicon spray once you have a new zip in place. I can only think of 1 thing it cant be used to lube up...

the metal oxidises forming combo of metal oxides and metal chlorides.. any acidic based cleaners would do the trick.

I have an old Dakine bag with BIG plastic teeth that looks like it will never have a problem.

I cannot, cannot, cannot,believe that that metal zips are still being used on surf bags/wetsuits/equipment.

vinegar-quickest and easiest.

Chips optional

Coke is the dogs dodahs for freeing crusty salty zips, just poor a good amount of coke on it leave for 5 mins and haypresto unstuck. Had to do it last week on a windsurf sail bag that hadnt been opened for over a year, well salted up, its now working as new.

Coke not diet coke.