SUCCESS! I'll call it Better Living Through Chemistry, but I'll never know for sure what worked. I took the advice from several helpful members and came to the boat with:
- Pot for boiling water
- Lazy Jack 5+ pound mini-sledge hammer
- White vinegar
- Rice vinegar
I didn't buy a coal chisel, but I had a socket screwdriver with a large bulb head, and I used that without the screw bit - which resulted in a blunter surface than a screwdriver. When I lost patience I just hit the slug directly with the sledge hammer. I tapped it from all 5 directions, though mostly forward and backward along the line of the boom track
Previously, going back a few weeks I had tried WD-40, liquid wrench, and recently PB Blaster. I looked for Kroil, but couldn't find it locally, and doubted it would do better than what I had tried - at least it was too similar to other failed approaches.
So I got to the boat, tried hitting with the Lazy Jack, then applied white vinegar, and tried again. I tried using my chisel substitute, but the U on the slide was a big enough target and the hammer alone seemed to deliver more force. I could see any misses were nicking the boom groove, and that kept me from getting wild with my swings. I boiled the water, hammered some more while waiting, then applied the boiling water, waited some, hammered some, etc. The vinegar & water intervals went on for a few repeats, with me doing other chores while waiting for soaking. Fluids would build up on the forward side of the slug, but drain off the back side - indicating a slight slope and a still good seal of corrosion under the slide. After 2-3 hours, I considered the rice vinegar, but when I saw the ingredients included salt, I considered that salt water was probably a key factor in the corrosion thus far, and stuck with white vinegar.
I realized I hadn't hit it full force, but the boom costs thousands and I have a dremel-type tool for planned deconstruction, so I gave it a bit more forceful hits and I thought it moved backward. So I hit it some more and someofabirch it moved again. So I kept it up and finally it popped backwards and I could slide it to the clew end of the boom - which is occupied by pulleys and the outhaul. But I could not slide it forward - it would get stuck again. There was a patch of greenish stuff, which showed whitish when I scraped it. I got out two sizes of screwdrivers and scraped for 15 or 20 minutes, and finally cleared enough of a path to slide it forward and out by the boom!
I'm not sure if this thing was brass or bronze, but my bet is bronze. It was soft enough that the hammer blows deformed the U atop the slide. That greenish powder must be copper corrosion and info on both metals is here:
Brass and Bronze
[trophy photos attached]
Next steps? The sail repair guy already put a stainless slide/slug in the aft-most spot. Based on what you all have suggested it seems:
- Get some solvent and clean that boom track
- Dremel polish the corroded spot
- Sand/polish the damaged paint on the boom
- coat the new stainless slug with something (Duralac?)
- Touch up paint the boom & damaged/polished portion of boom track
- Rub all slides/slugs with a bar of soap
- Install mainsail
- Win a mug racing with the extra .25kt from being able to adjust the outhaul!
Conclusions: Vinegar and boiling water worked better on this corrosion than penetrating oils. Heavy taps and medium force was enough and helped preserve the soft aluminum from Neanderthal damage.
Other note: adjusting the topping lift would have allowed me to have the fluid pool on either side of the slug. It turned out that the greater corrosion was on the front side, just by luck.