NaOH - sodium hydroxide.
If you dont have an easy source, then check drain cleaner (the powder stuff)
Be aware though - its truly vicious stuff when its dry. Wear gloves, wear eye protection, and watch out for dust (breathing it in)
What is nice, however, is that its (relatively) safe to dispose of down the drain - ie no complex disposal pathway.
I buy it in 25kg bags for about £24 (ie aprox £1/kg) but expect to pay more in small amounts. Id give you some if you were local, but theres no way im sending it through the post (!)
rebar will also weld no problem have welded it quite a few times
Parm might I suggest less reliance on welding the two ends together with the rebar and making use of the holes in the frame aswell seeing as you are going to be placing a LUMP of granite on it!
This mangle I re purposed had tie bars between the ends and I added some more