Friendly reminder: your soldering mat can be washed, so wash it

I let mine go way too long and paid for it. Mine sat on the bench for over a year without a wash and the flux build-up got so bad that small parts kept sticking to the surface instead of sitting where I placed them. Tried scrubbing it with soap like you describe and it helped, but the stubborn patches in the compartments needed a soft toothbrush and a second soak before they came clean. What finally got it back to normal was using a dab of baking soda on the brush for the worst spots. Gentle enough that the silicone didn't get scratched up. Once it dried I could actually see the printed markings under my parts again, which I'd forgotten even existed.

Now I wipe mine down every couple of weeks with a rag and keep the full wash for when it really needs it. Biggest trigger for me is lead clipping, all those tiny bits end up wedged in the divider corners and they're a pain to dig out once they mix with flux. I shake the mat off outside over the bin before washing now, saves the drain from a bunch of metal bits too. My mat is a cheap no-name one off Amazon but the same routine works fine on it.

The baking soda trick for the really stubborn patches is solid. I completely agree that trying to scrub hard usually just pushes the residue around. My mats have similar issues with hand grease and flux mixing, which creates a super sticky barrier.

What always helps me is just laying the mat out and giving it a good, long soak in just hot water and a little dish soap first. I let it sit for about ten minutes, and it seems like that breaks down the oils and lead clipping before I even bring out the brush. It makes the whole cleaning job much faster.

I find that the initial wipe down, rather than waiting for the deep clean, is significantly more effective for preventing structural bonding of the grime. If you wipe the surface promptly after any spatter occurs, you are mitigating the opportunity for the chemical solvents in the flux carrier to polymerize with the surface contaminants. My experiance shows that tiny amounts of oil or finger oils will effectively suspend in that flux residue, creating the sticky surface tension you are describing before it can even fully dry. For simple maintenace between full washes, I prefer to utilize a slightly damp microfiber cloth and a mild cleaner formulated for electronics. This allows for a more thorough agitation without introducing the risk of excessive water retention within the molded compartment channels, which is a crucial detail for preventing those micro-growth cycles of mildew.