I've used silica sand but had nothing but grief from the sand clogging due to picking up moisture from condesation in the tank.

Now I use a product called "Black Beauty" I think it's like a slag from the foundry process. I'm not sure but it doesn't clog like sand, cuts rust & paint and can be recycled. I put a big 20' x 50' sheet of black plastic on the ground to collect the stuff, then sift it through a screen for re use.

I get Black Beauty from the local ready mix / concrete products supplier.

Works GREAT!

As for getting the undercoating off. I place kerosene soaked rags on it, let it sit... then scrape it off with a putty knife.

Of course I do this all outside with good ventilation.