Making a keggel

So i'm planing on turning my 3 kegs into keggels. 1 for HLT, 1 for Mash, 1 for boil. I'm going to cut the hole in the top of the keg. Some still have alittle bit of beer in them. I was going to use my cutting torch to cut the hole and angle grinder to clean it up.

My question's are:

Do i need to empty the rest of the beer out? or will it catch the slag.

Should i put my sankey tap into the keg to release the pressure? or once theres a hole in the keg it vent on its own.

thanks
nick

You don't need to remove the beer out of the bottom, but make sure you release the pressure. Just put the keg on it's side and press in on the ball with a screwdriver.

Depends on how much you like the smell of rancid beer. When my friend helped me cut a couple with a plasma cutter, we pulled the valves, dumped the old beer, and then filled them almost all the way up with water to catch the slag.