Bit of self amalgy tape seal it?
I was thinking about aralditing it followed by some tape as you suggest. However, removed the duff batteries, cleaned up the PCB and fired it up on the bench last night. Very feint sound of a car alarm. Sounded like it was about 1/2 a mile away. Prodded around with a 'scope probe and it got a little louder. The acid from the batteries had got under some surface mount components on the board and, in this case, made a conductive path which was loading the alarm output on the CPU thus reducing the volume. I had scratched some of this away with the probe, hence it got louder!
Given that a car alarm needs to be reliable, and i doubt this one ever will again, I think I'll probably just get a new one - or not bother.
It did make me wonder what would have happened if it had failed the other way. A short between the supply rail and this signal would have turned on the transistor driving the sounder and probably burnt out the driver transformer or drained the car battery. Thinking about it, we have seen a few electrical fires originating from the power sounder area, and there's no evidence of any fuses or other protection circuitry in there.
Not sure what the supply to the sounder is fused at but it did make me think...
So, if your alarm starts to pack up unplug the power sounder sooner rather than later!
I have some photos which I'll post later if anyone's interested.
Kevin