Safety acid has to be checked and spiked after the first few hours, as the skin itself will change the level once its put into the pickle solution. Mix it as per Bruce's recipe, put the skin in, and then bring the pH back down below 2 the next day. How you handled the skin prior to pickle is where the best chance of slippage lies, besides the pH level. On a flat hide, a half hours worth of rough fleshing/beaming and then a few days of salt and draining is whats best.