This is a common problem and the "sludge" is crystalized resin.  If you heat it up slowly and stir it will go back into solution and is useable.  Make sure you get all the little bits dissolved.  It really makes a mess when you try and smooth it out onto a BID when there are little rocks of resin in the mix.  I would check the expiration date, but it is has most likely expired.  I think this tends to happen on older resin, and you probably only want to use it only on "non critical" parts.   It also helps to keep the resin in a warm box - especially with winter coming.  If you keep it around 75-80, it should stay in solution.