I dont know but makes sense since evrything else preds use aginst them will eventually burn or melt from the Acid or just maybe the Preds have the Technology to look into the Aliens genetic make up and mix it in with their type of metal weapons to use aginst them like in AVP
(all acid proof armor)2Writsblades(acid proof),comitstick(Acidproof),2shurikens,Smart Disc(Acid Proof),speargun,plasmacaster, netgun,cermonial dagger,and 2elder daggers(acidproof,and there from Pred2)