Thorough is key. Don't be shy with the spray, blast the hell out of it and hope enough wicks up under the plastic housing with the contacts. If problems persist, overhauling is always an option, but you need patience and some basic soldering/fidgeting skills.
1. Remove all solder from the pins with a desoldering gun, or a normal iron with a sniffer/bulb/wick. Ensure all pins are completely detached from the board. A few pins will be bent over, in order to keep it on the board before it was wave soldered. It's easy to lift pads on these, so heat the pin as you straighten it, unless you're 100% sure no solder is left before straightening them.
2. Straighten the case tabs enough until the contact board can come out. There is a spring under tension connected between the case and the slider bracket, which can be tricky to re-install, so take note of the endpoints it is anchored to.
3. Carefully remove the contact leafs on the contacts and set aside. They are almost always of a corrosion-resistant alloy and should look fine.
4. use whatever gunk cleaner you desire and get that contact board nice and shiny clean. Don't use abrasives, you will ruin the thin coatings of metal.
5. Give the leafs a good spraying and put back onto the contact board. This is a very delicate process to avoid bending them, so use a head magnifier and lots of light if need be. They should all align with the pockets on the slider bracket, so it just falls back into place flush against the contact board.
6. Lay the spring on top of the plastic slider with one end attached to it, and slide the whole thing back up into the metal case. Bend the tabs back into place and use a pick to pull the spring and anchor the other end to the housing.
7. Solder back into place on the board.