I just fixed a leaking tank that had 'slosh' in it. I drilled four corner holes and used a cut off wheel to cut the openings for each rib bay in the back cover of the tank. I deburred the holes to reduce bleeding, I drilled out the leaking rivet. We got to work with scotchbrite pads and polysufide remover. It took a few days to get the slosh out, and lots of rinse water.
eventually it was clean enough. I cut the covers for the openings then drilled and cleco'd them on. I made the anti rotation bracket for the fuel pick up, and removed the abandoned remnants of a flop tube.
The slosh was not well attached. It was thick in places, thin in others, loose and flopping in some, and required a lot of diligent scrubbing to remove in other places. Most of this was done by my wonderful wife who wanted her plane back.
When everything was clean and ready, I weighed & mixed baseball sized batches of proseal (or flamemaster) and applied it with a nitrile gloved hand over all the seams & rivet lines. I sealed and re-riveted the leak then also the tank suction line repair and finally the cover plates, which I pop riveted on with the same rivets called out to assemble the tank in the first place.
The tank fit right back on and does not leak.
You can do it too.
Otherwise, Curtis' Post #7 applies.
The "In Between" would be to remove the tank, remove the big round cover on the inner rib that supports the suction tube and the tank level sender, and inspect / clean as required. Then put a decent sized wire mesh strainer on the suction tube. This assures that the suction tube anti-rotation bracket has been installed, the inner bay of the tank is not full of loose 'slosh' bits, and you have a safe fuel strainer at the beginning of the fuel path thru your engine.