Not booting normally without SMC reset every time?

Hi, Marc here. So a friend asked me to look at his macbook pro, I've worked on macbooks in the past and have been able to resolve most issues, including bad HDD cable, battery, fan, speakers etc.

This machine is different. He spilled water on it and then tried to turn it on! So I opened the machine up, removed the logic board and cleaned it all up. There was some corrosion that brushed out with an old tooth brush. I got the machine to boot by connecting the power adapter then holding down the power button until the boot light flashes, then stays on, I release the power button and the machine boots, but starts with the SMC rebuild progress bar - the problem is I have to do that every time I want the machine to boot. If I don't, the boot light on the front flashes briefly and the machine doesn't turn on.

I've replaced the battery - which wasn't showing as available in the system once booted, the new battery is charging correctly and I can start the machine up when not connected to power using the modified process I mentioned above.

The OS is also very slow when I get in, no loud fan, but extremely slow response times.

My question is - what can I do to prevent the SMC reset every time to get the machine to boot normally? I assume the solution will also solve the slow system symptoms somewhat (alliteration anyone?).

I've scoured the internet, tried the several different solutions for resetting PRAM and SMC, to no avail.

I also plan on reloading the OS once I get the boot issue sorted, this will hopefully help with speed issues as well. Looking forward to your thoughts!

Thanks in advance!