Its not acid its base. Use vinegar and an brush. Apply vinegar for some time and gently brush.
Base is nasty and can get under the copper trace and break it. As well as get into middle layers of the board.

I think that some trace is in fact broken where traces are blue...

Vinegar like the usual usual cooking red vinegar right? 😁

Yeah. When acid and base mix up they get neutral. A few minutes should be enough. Clean with water after.

Well, cleaning up a 386 motherboard with vinegar I discovered bad traces but also some disk ceramic orange rounded component opened in half, they are labeled as "C6" for example so they should be capacitors but they didn't have any numbers into it. How can I know with what to replace it?

I've seen that on other positions on the mainboard there're others with 103 onto it (so 0,01uf ?) but the voltage? The capacitor a small like 3/4mm.

386SX wrote:

I've seen that on other positions on the mainboard there're others with 103 onto it (so 0,01uf ?) but the voltage? The capacitor a small like 3/4mm.

The voltage isn't that critical. There is no higher than 12V on the board anywhere so a 16V cap would be enough

Ok! I will buy both battery and some 0.01uf capacitors to replace but I've doubts this car will boot..

correct , ceramic caps marked 103 are 0.01uF , their usual voltage rating is 50v or 63v - so no need to pay attention to it for a mobo