The cells sealing dried nicely. No leakage. The bigger cell doesn't look rugged as 6" x 6" cell. I proceed with cleaning process with 6" x 6" cell. I didn't fix the cell into container at this moment.
I'm using power supply with adjustable voltage range between 9.5VDC to 13.54VDC and can handle upto 40A. Placed the cell in open area and pour in old electrolyte (NaOH solutions) till top. Suck out some electrolyte using syringe from each cell compartment. Apply lowest voltage first. When water rises to top then suck with syringe again. Increase voltage by stages and repeat excess electrolyte removing process until the meter reads max voltage 13.54VDC and 2A.
I wanted to get around 5A with 13.54VDC. Pour in electrolyte from the cell into glass container and add more NaOH pallets. Refill back the electrolyte into cell. Did the same process to maintain water from overflow. I got 5A with 13.54VDC.
Wanted to check how much water level needed to avoid overflow at 5A. Power off. Water level was 10mm below the top. I tried to reverse polarity to clean the other side of plates. I saw meter reading rose to 6.5A and electrolyte overflows. Why?

The water reduces faster than expected because of vaporisation. Touched the end plates, slightly warm. Replaced water losses with distrilled water. When the bubble foam reaches top and start flowing to other cell, meter readings goes higher (shorting) and comes back to initial value if the foam stays within its cell compartment.
Power off and let the electrolyte in the cell overnight. I dare not leave the cell powered overnight. Today morning, I notice the water level at end plates reduced much lower compared to middle cells. I refill back with distrilled water.
Today, I will fix the cell inside container and refill with new electrolyte for conditioning. I will set around 1-2A and power the cell for maybe 24hours. Also I will measure gas production after the cleaning process.