Here's some experience info which may be of some help to others with similar problems.
A slow-weeping leak in the left wing tank, appeared on the lower flange of the front wing spar, in the cockpit, accumulated on the floor.  
By draining the tank in stages I found that with fuel at about 8 gallons -- half full-- the leak stopped.  Concluded the leak to be through the front spar web, and in upper half... but the leading edge conceals and prevents access to the spar web. .. so I couldn't tell where it was span-wise, although I made a vacuum and manometer apparatus to try to hear leak hissing with my stethescope... which I hope I never have to use.
 Therefore no alternative but to pull the wing off, and prepare for sloshing.

Searching for an appropriate sloshing compound was frustrating.  One 3M manufacturer stated "No data is available for directions to apply this product", or words to that effect.  Others stated 'not for aircraft use', but gave directions for application.  Vans blogs and some others  warned against use if the fuel had methanol or diesel.  One was recommended for use in Lancairs, a 3-gallon 2-part kit costing about $300.  Some were one-part rubber compound which cured by evaporation of MEK vehicle, PR-1005-L, which I elected to try, although directions for use were lacking, for one quart for almost $200.  However the cheaper, similar slosh for a motorcycle tank sloshing product,  gave the most directions.
I finally decided on a procedure which I hope worked.  I assumed one quart would be enough for  my 16+ gal. tank.
The similar cheaper product suggested a quart for only 12 gal. tank capacity, and indicated slosh could be thinned by about 25%R MEK added, and applied twice for two thin coats would reportedly would dry faster than one thick coat.  Its recommended directions were to first flush the tank with MEK, purpose not explained, but I think the purpose was to absorb up to 3 grams of any water remaining trapped in the tank first. Then the slosh applied.

I did the MEK slosh first, then drained the MEK into a container, through the tank fuel outlet; then blow-dried out the tank by using the outlet of my Shopvac, for an hour.  Then again plugged the outlet, and poured slosh in though the filler cap, and first rocked the wing horizontal for the outer tank bay (tank is divided by a rib in the tank center with three or four holes in top of the rib and bottom).
Then stood the tank on the leading edge to coat the outer part of the front spar web.  Then laid the wing flat and tilted it toward the root, to let the slosh drain through the rib holes to the inner bay, and repeated the horizontal rocking, then raising it to its leading edge to coat the inner part of the front spar web. Then tilted toward the trailing edge to coat the rear spar web, then tilted toward the wingtip again to flow the slosh outboard and did the same to the outer part of the rear spar web.
Then the wing was tipped toward the root after unplugging the fuel outlet, and the remaining slosh drained out.
I was happy to see that about half a quart of slosh drained out, and I returned it to its container for a second coat if needed, after testing for leaking.  
The directioons for the similar slosh compound had suggested letting the slosh stand too cure in 48 hours, or to start curing for an  hour before blowing air through it to speed drying/curing. So i stood the wing on its leading edge for an hour before starting the blower again and ran it for an hour; then returned the wing to horizontal and let it rest with vent, outlet and inlet open, where it is now.  I did the sloshing in this order because I had no way of knowing how much slosh was going to be use du,p to coat the tank, and wanted to prioritize the root end of the front wing spar.

I suspected the drying would be slow because the hangar temp was abut 50-55F, and a sloshing compound gave specs for 77F..  I had a sample of the slosh in an open container, and brought that home to observe the curing progress... at the same temperature,  in my shop. Where the slosh coating is thin it dried fairly fast... in less than an hour; but where it puddled about .060 to .090 thick, it has taken two days to gradually dry to a thinner but still softer consistency.

Today i thinned the remaining pint of slosh with 25% MEK, and poured a sample (sample similar) amount on top of the original sample, to test whether it would adhere, and to see the drying time.
It dried fairly fast at 55F, and appears to merge smoothly onto the first sloshing compound, so now i plan to repeat the same sloshing procedure with thinned slosh, as soon as the temperature get up to about 55 to 60F, to try to be sure the leak is closed, and any future similar leak prevented.

Its hard to understand why this product could not have had Directions for application, perhaps suggesting a temperature for application, and alternate methods or preferred methods of application, prepping for same, re-coating, thinning the slosh, etc., for corporations like PPG and 3M.

Any comments welcome.
To be continued ....
Terrence
L235/320 N211AL